memory: snapshot — U5 released at v0.3.0, and the index goes two-tier

The two dated log sections had never been split, so every one of their 29
entries sat inline and the startup index had grown to 372 lines — which is
the cost the two-tier scheme exists to remove, paid on every session that
reads the file. 27 entries were over threshold. All 29 now have a detail
file under persistent-memory.d/ and a one-line index entry that routes
rather than restates. Index: 372 -> 93 lines.

No archival. The soft cap fired, but every entry in this repo is dated
2026-09-21 or later, so the under-14-days guard held all of them back — and
the split alone took the index well under the target without moving
anything out of the active file.

The in-flight section is rewritten for the post-release state: nothing is
in flight, no gate is outstanding, and the next unit is explicitly recorded
as the operator's undecided call rather than as a plan. The session's
recommendation (U4, on three grounds) is written down so it does not have
to be re-derived, alongside the two alternatives and why they are
alternatives.

Two dated predictions are carried forward with their dates and their
instruments: the U5 adoption re-measure on 2026-09-29, which already reads
3 of 24 announced and 2 with a why from peers told nothing, and the
.forever re-count a fortnight AFTER U4 lands, which is U4's own success
criterion and is destroyed by running it early.

persistent-memory.d/2026-09-21-u7-section-premise-half-wrong.md

@@ -0,0 +1,19 @@
+# U7's section premise is half wrong
+
+_2026-09-21 · booth_
+
+**U7's section premise is half wrong, and it is the half that
+matters** — found by re-measuring `~/booth-data` rather than trusting the IA
+doc. The IA says sections come from subfolders that already exist on disk;
+true, but **every booth that actually needs navigation is flat**:
+`pancake-v3-full` (270 items, 0 subfolders), `pancake-v4-full` (270, 0),
+`sindra20-engines` (98 items + 99 caption sidecars, 0), `sindra-finalists`
+(86 + 87, 0). Subfolders exist on exactly two booths — `pewpew-ui-brief` (7,
+nested to `_ds/powerpellet-design-system-<uuid>/preview`) and `dfa-concepts`
+(1) — and **both are reports**, the job where grid navigation matters least.
+So sections stay worth shipping and `Item.section` stays right, but they are
+**not** "most of the navigation fix": the rail, the filters and grid keyboard
+are all of it. Worth noting for whoever writes U7: `sindra20-engines` encodes
+its structure in the **filename prefix** (`b2-s1-<subject>-<seed>`), which is
+where a grouping heuristic would actually pay. The IA doc's claim about what
+sections buy needs a line struck — not yet edited.