memory: U7 re-measured before scoping — pre-work only, no unit started

The standing instruction is to re-count the booths before scoping U7. Done
against the live 19-booth set, so the scope call is a short read rather than an
investigation.

Two findings. Sections are worth zero and it is now measured twice: not one of
the eleven gallery booths has a subdirectory, and the only two booths that do
are both reports, the job where grid navigation matters least. And the grouping
signal is in the filename rather than the tree -- stripping a trailing digit-run
yields 5 to 16 sensible groups on four of the five large galleries and
degenerates to one group on the fifth, while the competing split-on-second-
hyphen heuristic is useless everywhere.

The sizing case has also moved: the unit was scoped against 270-item booths and
the largest gallery is now 81 items / 40 images.

No U7 code and no U7 contract. The scope direction is the operator's call.

persistent-memory.d/2026-09-22-u7-remeasured-before-scoping.md

@@ -0,0 +1,70 @@
+# U7 re-measured before scoping — sections are dead, filename prefixes are not
+
+_2026-09-22 · booth_
+
+**Pre-work, not the unit.** The standing instruction is "re-count the booths
+before scoping U7". Done, on the live set (19 booths). No U7 code, no U7
+contract — this exists so the scope call is a thirty-second read.
+
+## The set as it actually is
+
+| booth | items | images | subdirs | shape |
+|---|---|---|---|---|
+| `miranda-is` | 92 | 0 | 0 | report |
+| `sindra-bakeoff` | 81 | 40 | **0** | gallery |
+| `sindra-corpus-v1` | 66 | 66 | **0** | gallery |
+| `sindra` | 61 | 30 | **0** | gallery |
+| `sindra-sfw-pool` | 59 | 59 | **0** | gallery |
+| `sindra-nude-pool` | 42 | 42 | **0** | gallery |
+| `pewpew-ui-brief` | 34 | 1 | 7 | **report** |
+| `dfa-concepts` | 21 | 14 | 1 | **report** |
+| …11 more | ≤19 | | 0 | |
+
+## Finding 1 — sections are worth ZERO, and this is now measured twice
+
+**Not one gallery booth has a subdirectory.** Zero of eleven. The only two
+booths with subfolders are both **reports**, the job where grid navigation
+matters least, and `pewpew-ui-brief`'s seven subdirs hold one image.
+
+The IA doc calls sections "most of the navigation fix". On this set they are
+none of it. Cutting `Item.section` rendering from U7 costs nothing measurable.
+(`Item.section` already exists from U1 and stays — this is about whether U7
+builds a section RAIL, not about deleting a field.)
+
+## Finding 2 — the grouping signal is in the FILENAME, and it pays
+
+Tested two heuristics against every large gallery. Strip a trailing digit-run
+from the stem and group on what remains:
+
+| booth | images | groups | verdict |
+|---|---|---|---|
+| `sindra-corpus-v1` | 66 | **16** | useful |
+| `sindra-nude-pool` | 42 | **12** | useful |
+| `sindra-sfw-pool` | 59 | **10** | useful |
+| `sindra-bakeoff` | 40 | **5** | useful |
+| `sindra` | 30 | **1** | **degenerates** |
+
+Specimens: `00-sheet-c1-market-noon.png`, `ac01.png`, `a01.png`,
+`flag-rear.png`. The competing heuristic — split on the second hyphen — is
+useless everywhere (59 "groups" from 59 files).
+
+So a prefix heuristic pays on **4 of 5** large galleries and collapses to one
+group on the fifth. **That is a filter/grouping affordance, not a section
+rail**, and it must degrade gracefully to "one group" rather than render a
+useless single-section rail.
+
+## What this implies for the scope, stated as a recommendation not a decision
+
+U7 as written is four things: sections, a sticky rail, filters, grid keyboard.
+The measurement says **drop sections, keep the other three**, and consider
+prefix-grouping as the thing sections were supposed to be — with a stated
+degenerate case.
+
+⚠ The sizing case has also changed: the unit was scoped against 270-item
+booths and **the largest gallery is now 81 items / 40 images**. Everything
+about virtualization stays parked ([[2026-09-21-ia-and-v1-gate-landed]] names
+it); at 66 images a lazy grid is fine and measuring it first is the rule.
+
+**The booth set churned again during this session** — `sindra-sfw-pool` (59
+images) appeared and the `pancake-*` set went. Re-count again before writing
+the contract; do not trust this table either.

persistent-memory.md

@@ -78,6 +78,7 @@ _As of 2026-09-22:_
 
 ## Recent decisions
 
+- `[2026-09-22]` **U7 re-measured before scoping — sections are dead, filename prefixes are not** — PRE-WORK ONLY, no unit started; read before writing U7's contract → `persistent-memory.d/2026-09-22-u7-remeasured-before-scoping.md`
 - `[2026-09-22]` **The last open defect closed, and building its falsifier found another** — the wrong-shaped answer fixed at `_hydrate`; `_safe_fragments` lost its natural trigger and its handler could not survive the failure it handled → `persistent-memory.d/2026-09-22-a-wrong-shaped-answer-500s-the-gallery.md`
 - `[2026-09-22]` **U6 released as `v0.6.0` — benches, and the number that was two defects** — six of seven v1 units landed, NOT PUSHED → `persistent-memory.d/2026-09-22-u6-benches-released.md`
 - `[2026-09-22]` **Three cold panels on one unit, and what each lens could only see alone** — READ BEFORE DECIDING TO SKIP A GATE; all five passes found something the others structurally could not → `persistent-memory.d/2026-09-22-three-cold-panels-on-one-unit.md`