memory: the cache that aged the thing it cached

Two lessons from the thumbnail work, the second of which nearly shipped.

We parked progressive loading on a count of images and the cost was in bytes.
'Measure the real booth before optimising it' was followed and still gave the
wrong answer, because we measured the dimension that was easy to measure rather
than the one the user feels.

And a cache living inside the thing it describes can age that thing. Excluding
every path under the cache dir passed its own test and was still wrong: creating
the directory touches the BOOTH's own mtime, which is what _newest_mtime seeds
from. The contents were excluded; the existence was the leak. Had it reached the
Desk, one index load would have pushed every booth's expiry out and the TTL
would never have fired again.

persistent-memory.md

@@ -199,6 +199,7 @@ _As of 2026-09-22:_
 
 ## Recent decisions
 
+- `[2026-09-23]` ⚠ **The cache that aged the thing it cached** — thumbnails 77.5MB→0.78MB; READ BEFORE PARKING ANYTHING ON A MEASUREMENT (we counted images and the cost was in bytes), and BEFORE PUTTING A SERVER-WRITTEN CACHE INSIDE A BOOTH (excluding its contents does not stop it aging the booth) → `persistent-memory.d/2026-09-23-the-cache-that-aged-the-thing-it-cached.md`
 - `[2026-09-23]` ⚠ **The probe that nearly dismissed a live injection vector** — the link board rendered `javascript:` hrefs; READ BEFORE TRUSTING A NEGATIVE RESULT FROM AN OBVIOUS PROBE, and before assuming an existing scheme check is the guard you are looking for → `persistent-memory.d/2026-09-23-the-probe-that-nearly-dismissed-a-live-vector.md`
 - `[2026-09-23]` ✅ **The bug-hunt panel found six defects and five vacuous falsifiers** — READ BEFORE BUILDING ANY FRAGMENT ANCHOR (browsers match raw before decoded, so both sides must be encoded), and before trusting a well-commented diff's guards → `persistent-memory.d/2026-09-23-the-bug-hunt-panel-and-five-vacuous-falsifiers.md`
 - `[2026-09-22]` ✅ **v1.0.0b1 — the v1 target staged as a beta, and a version that was two copies** — READ BEFORE DERIVING A VERSION FROM `importlib.metadata` HERE; it reports a different artifact, and `booth/__init__.py` turns out to be stdlib-only → `persistent-memory.d/2026-09-22-v1-staged-as-a-beta-and-a-second-copy-of-the-version.md`