feat(u7): filename groups — the last v1 unit, and a table that did not reproduce

Completes U7 with its fourth component: a jump-to-group rail derived from
filename prefixes, replacing the subfolder sections ROADMAP named. The scope
departure was ratified by the operator 2026-09-22; this commit deletes
test_no_group_rail_is_shipped_yet, the guard that held it back, in the same
change that builds what it guarded against.

All seven v1 capabilities are now landed. The 1.0 cut is a decision, not a
dependency, and it is the operator's — no version bump here, because a commit
is not a release.

THE RULE CHANGED AT IMPLEMENTATION, ON MEASURED GROUNDS. The contract specified
`strip ONE trailing run of digits`; run against the live set that yields 24
groups for sindra-bakeoff's 40 images and 27 for sindra's 30 — a rail with a row
per tile — because it keys on the END of the stem, where the instance number
lives. The contract's own table claimed 5 and 1 for those two booths and neither
reproduces; the numbers are reachable only by two OTHER heuristics, so the table
that justified the design was assembled from more than one rule. Its own worked
example contradicts it in plain sight.

The shipped rule keys on the first separator-delimited segment, where the family
lives, destemming only when the stem has no separator at all — so `ac01` -> `ac`
while `v30-seed8302` and `v35-seed8302` stay apart. Re-measured across all 17
live booths; the table is in the contract.

INV-3 GAINED ITS SECOND DEGENERACY. The contract guarded one group for
everything (sc-iso-spread: DSC0001-DSC0006). The live set's actual failure is
the opposite — pewpew-ui-brief yields 23 groups for 34 items, dfa-concepts 13
for 20 — and the contract as written would have shipped a rail that is a second
copy of the grid. The rail now renders only when grouping is informative: two or
more groups, and the middle group holding more than one item. That predicate
gets all 17 booths right.

Grouping is a VIEW. The grid stays sorted(rel) and the zoom ring stays that
order filtered to images; the group fixture interleaves across subdirectories
precisely so a (group, rel) re-sort goes red. Groups are derived from the
RENDERED list, not the full gallery, so no anchor points at a filtered-out tile.

booth/items.py       _group_of + Item.group, derived in the resolver (INV-1)
booth/app.py         _groups() builds the rail rows; build_gallery carries it
booth/templates/     the rail-groups nav and its CSS
tests/               +16 tests; 639 green

Every new falsifier was proved by running its defeating change (12/12). Three
were vacuous first time out: one fixture's positional order happened to be
alphabetical, one assertion miscounted elements, and the harness itself
certified a broken test twice — no green baseline, and byte-identical mutations
silently defeated by the pyc cache's one-second mtime granularity.

ROADMAP.md

@@ -1,7 +1,9 @@
 # The Booth — roadmap
 
 Design: [`docs/design/information-architecture.md`](docs/design/information-architecture.md).
-Current version: `0.6.1` (U1 through U6 landed; extracted from eshpfi 2026-09-21).
+Current version: `0.6.1` (**U1 through U7 landed — every v1 capability is in**;
+extracted from eshpfi 2026-09-21). **The 1.0 cut is now a decision, not a
+dependency**, and it is the operator's: a major bump needs his approval.
 
 ## v1 target
 
@@ -16,26 +18,29 @@ defect — not a wish. The measurements are in the IA doc.
 | 4 | ~~**Derived lifetime**~~ — **landed `c3a97c1`, released `v0.4.0`** | 70% of booths on the `.forever` escape hatch (54% when first counted) | U4 |
 | 5 | ~~**Self-announcing booths**~~ — **landed `c015a91`, released `v0.3.0`** | job 5 had no home, so it lived on the link board as 145 dead rows | U5 |
 | 6 | ~~**Benches**~~ — **landed `1c3ce5d`, released `v0.6.0`** | 69% link-board rot (re-measured: 178 booth rows + 8 bench re-posts) | U6 |
-| 7 | **Navigation** — ~~sections~~ **filename groups**, rail, filters, grid keyboard | one flat wall; 0 of 11 galleries have subfolders, so grouping comes from the filename | U7 |
+| 7 | ~~**Navigation**~~ — ~~sections~~ **filename groups**, rail, filters, grid keyboard — **landed, unreleased** | one flat wall; 0 of 11 galleries have subfolders, so grouping comes from the filename | U7 |
 
 Ordering is dependency-driven, not priority-driven: **U1 → U2 → {U3, U4, U5} →
 U7**, with **U6 independent** of all of them (different storage, different
 surface) and therefore the safest thing to land first or in parallel.
 
-**U1 through U6 are landed.** **U7 is the last unit before the 1.0 cut** — its
-only dependency was `{U3, U4, U5}` and that closed with U3.
+**ALL SEVEN UNITS ARE LANDED.** U7 closed last; its only dependency was
+`{U3, U4, U5}` and that closed with U3.
 
-⚠ **Before starting U7, read
-`persistent-memory.d/2026-09-21-u7-section-premise-half-wrong.md`, and re-count
-the booths first.** Half its premise is already known to be wrong — every booth
-that actually needs navigation is FLAT — and the booth set churned again on
-2026-09-22: the four large booths U7 was sized against (`pancake-v3-full` and
-`pancake-v4-full` at 270 items, `sindra20-engines`, `sindra-finalists`) have all
-been swept. The largest live booth is now `miranda-is` at 92 items, flat. Two of
-23 booths have subfolders (`pewpew-ui-brief`, `dfa-concepts`) and **both are
-reports** — the job where grid navigation matters least. Sections, one of U7's
-four named components, buys close to nothing. The rail, the filters and the grid
-keyboard are the unit.
+⚠ **What U7 actually shipped is not what this row first described, and the
+difference is measured.** Sections were dropped for filename-prefix groups
+(operator-ratified 2026-09-22) because zero of eleven gallery booths have a
+subdirectory. Then the *grouping rule itself* changed at implementation: the
+contract's `strip a trailing digit run` yields 24 groups for `sindra-bakeoff`'s
+40 images and 27 for `sindra`'s 30 — a rail with a row per tile — because it
+keys on the end of the stem, where the instance number lives. The shipped rule
+keys on the **first separator-delimited segment**, where the family lives, and
+gives 4 and 2. The full re-measurement across all 17 live booths is in
+`docs/contracts/u7_navigation.contract.md`.
+
+**The v1 target is met.** What remains is a release decision the operator owns:
+cut `1.0`, or take a `0.7.0` staging release first. Nothing in the code is
+waiting on it.
 
 **U5's adoption is a measured prediction, not a finished result**, and it is
 TWO predictions rather than one. The operator declined a fleetwide announcement
@@ -85,6 +90,8 @@ Where it already binds, and what the rule is in each case:
 | legacy ask import | `(mtime, name)`, which is the order `list_asks` gave them |
 | link board rows | pinned first, then newest-first |
 | a booth's announcement | not a collection — one flat record per booth, nothing to order (U5) |
+| **groups among themselves** | **the position of each group's first member in the rendered sequence** — `sorted(rel)` narrowed by the filter, never re-sorted. Walking the rendered list once into an insertion-ordered dict IS the rule, so there is no second sort to drift from it (U7) |
+| **items within a group** | not a separate order — a group is a label on a tile, not a container. The grid stays `sorted(rel)` and groups interleave in it freely (U7) |
 | the bench registry | `(state rank, name casefolded, id)` — live before promoted before retired, then alphabetical, with the id as a TOTAL tie-break so two benches sharing a name cannot swap (U6) |
 | the link board's dead marker | not an order — a per-row stamp read from the existing `order_for_display` sequence, so marking cannot move a row (U6) |
 | embed anchors in a verbatim report | **document order** — what `querySelectorAll` yields, so the author's markup decides (U3) |
@@ -101,18 +108,18 @@ surfaces (index card, booth header, marks page) render through ONE macro
 precisely so they cannot disagree, which is the same property stated for
 ordering: one rule, one place, every surface reading it.
 
-**U7's group ordering is SETTLED** (operator, 2026-09-22): groups order by the
-position of their first member in `sorted(rel)`, so the rail reads in the same
-direction as the grid. Subfolder sections are dropped in favour of
-filename-prefix groups on measured grounds — zero of eleven gallery booths have
-a subdirectory. Compare pairing rides with compare mode into v1.1.
+**U7's group ordering is SETTLED and SHIPPED** (operator, 2026-09-22): groups
+order by the position of their first member in the rendered sequence, so the
+rail reads in the same direction as the grid. Subfolder sections were dropped
+in favour of filename-prefix groups on measured grounds — zero of eleven
+gallery booths have a subdirectory.
 
-Formerly undecided, retained for the record: **U7's
-section ordering and its compare pairing** (sections need a stated order among
-themselves, not just within; pairing by filename needs a rule for what happens
-to an unpaired file). **U6's bench listing is settled** — the row above.
-Compare pairing is parked to v1.1 with compare mode itself, so U7 carries one
-undecided rule, not two.
+**Nothing in this table is undecided any more.** The two U7 rules that were
+(section ordering among themselves, compare pairing) resolved differently:
+section ordering is MOOT, because U7 renders no section rail — `Item.section`
+still exists and is still derived, it simply has no ordered surface. Compare
+pairing rode into v1.1 with compare mode itself. **U6's bench listing is
+settled** — the row above.
 
 The test for any new ordered surface: *can you write the rule down in one line?*
 If not, it does not have one yet.