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.

persistent-memory.md

@@ -26,16 +26,28 @@ _As of 2026-09-22:_
   recs").** Four of five settled and executed; one blocked by the permission
   layer. Nothing is waiting on him. →
   `persistent-memory.d/2026-09-22-operator-ruled-on-the-open-five.md`
-- **U7 IS THE ONLY WORK LEFT, AND IT IS NOW UNBLOCKED.** The three ratified
-  components (rail, filters, grid keyboard) are landed at `a306e2d`. The fourth
-  — **filename-prefix groups replacing subfolder sections — is APPROVED** and
-  NOT yet built. That is the next session's first job.
-  → `persistent-memory.d/2026-09-22-u7-three-quarters-and-one-ruling.md`
-- ⚠ **`tests/test_navigation.py::test_no_group_rail_is_shipped_yet` IS NOW A
-  STALE GUARD.** It exists to stop an UNAPPROVED group rail; the rail is
-  approved, so whoever builds it must DELETE that test in the same commit. It
-  will otherwise read as a real invariant and block correct work.
-- **623 tests green. Tree clean and pushed. Deployed; 19/19 booths 200.**
+- ✅ **U7 IS LANDED — ALL SEVEN v1 UNITS ARE IN.** The fourth component
+  (filename-prefix groups) is built; `test_no_group_rail_is_shipped_yet` was
+  deleted in the same commit, as required.
+  → `persistent-memory.d/2026-09-22-u7-landed-and-a-table-that-did-not-reproduce.md`
+- 🔶 **THE 1.0 CUT IS NOW A DECISION, NOT A DEPENDENCY, AND IT IS HIS.** The v1
+  target is met. A major bump needs explicit operator approval; nothing in the
+  code is waiting on it. The open fork: cut `1.0`, or stage a `0.7.0` first.
+  **Not bumped — the work is committed as commits, which are not releases.**
+- ⚠ **U7'S CONTRACT CARRIED A MEASUREMENT THAT DID NOT REPRODUCE**, and it was
+  the number the scope departure rested on. The stated rule gives 24 and 27
+  groups where the table claimed 5 and 1; the table was assembled from two
+  different heuristics. **A cold contract-review panel cannot catch this** — the
+  artifact is internally plausible. Re-run any measurement a contract's scope
+  rests on before implementing it. Same detail file.
+- ⚠ **A MUTATION HARNESS NEEDS A GREEN BASELINE AND CACHE DEFEAT**, or it
+  certifies falsifiers without running them. Both defects bit in one session.
+  → `persistent-memory.d/2026-09-22-a-mutation-harness-that-certified-a-broken-test.md`
+- **639 tests green; 12/12 new falsifiers mutation-proved. Deployed; 21/21
+  booths 200.** ⚠ The set churned again mid-session (19 → 21).
+- 🔶 **A bug-hunt panel is IN FLIGHT** — heid thread `01M368G2Y0JMTJ2T7M3JMTXV5Z`,
+  dispatched 2026-09-22 21:31 PDT over the U7-groups diff. If its reply has not
+  been consumed, drain `/althing:inbox` and triage before treating U7 as closed.
 - 🛑 **STANDING RULING — NO ANNOUNCEMENTS OUT OF THIS REPO, AND THE OPERATOR
   SENDS THE EVENTUAL ONE HIMSELF** (operator, 2026-09-22). Verbatim: *"no
   announcements until the entire arc is done, and even then i'll do it myself."*
@@ -72,6 +84,8 @@ _As of 2026-09-22:_
 
 ## Recent decisions
 
+- `[2026-09-22]` ✅ **U7 landed — and the number that justified it did not reproduce** — all seven v1 units are in; READ BEFORE TRUSTING A MEASUREMENT INSIDE A CONTRACT, and before assuming a degeneracy guard covers the degeneracy you actually have → `persistent-memory.d/2026-09-22-u7-landed-and-a-table-that-did-not-reproduce.md`
+- `[2026-09-22]` ⚠ **A mutation harness certified a broken test, twice, for two reasons** — no green baseline, and the pyc cache silently reverting same-size mutations; READ BEFORE WRITING ONE → `persistent-memory.d/2026-09-22-a-mutation-harness-that-certified-a-broken-test.md`
 - `[2026-09-22]` 🛑 **STANDING: no announcements out of this repo until the arc is done, and he sends that one himself** — verbatim *"no announcements until the entire arc is done, and even then i'll do it myself."* Stricter than the house broadcast gate: the send is not the agent's to make, so **asking is also out of scope**. The drafted 17-handle note is REASSIGNED to him, not blocked — see the in-flight row above; do not raise it again.
 - `[2026-09-22]` **The operator ruled on all five open items at once** — four executed incl. the first push; the broadcast was blocked by the permission layer and is drafted at `docs/pending/` → `persistent-memory.d/2026-09-22-operator-ruled-on-the-open-five.md`
 - `[2026-09-22]` **U7 is three-quarters built and blocked on one word** — the ratified three landed; the sections-vs-groups departure is NOT built and is the operator's call, tracked at `docs/contracts/u7_navigation.contract.md` → `persistent-memory.d/2026-09-22-u7-three-quarters-and-one-ruling.md`