fix(size): the v1.9.1 residual was npm's own cache, not the platform binaries

v1.9.1's @esbuild prune fixed the 431 MB size-gate failure but still shipped
+131 MB compressed over v1.8.14, nearly all of it in the pi/extensions install
layer (87 -> 206 MB). That leftover was filed as an open item with an explicit
hypothesis — the @mariozechner/clipboard-* family, same npm 11 behaviour, a
different package — and an explicit warning that the hypothesis was not a
measured cause. Measured now, after recreating onto v1.9.1, and the hypothesis
accounted for one sixth of it:

  +110 MB  /root/.npm/_cacache  (35.2 -> 145.3 MB)   the build's npm cache
  + 21 MB  clipboard foreign platform packages, both install sites
  = 131 MB  i.e. the whole delta, no unexplained remainder

Method, since there is no docker CLI inside the container: pulled both variant
layer blobs straight from the registry with a token + manifest + blob fetch and
listed the tarballs (29 789 vs 29 889 entries, 270.8 vs 401.9 MB uncompressed),
then aggregated per package. The file COUNT barely moved, which is what said
"few large files", not "npm installed more packages".

  - purge_build_caches: npm cache clean --force + rm -rf /root/.npm, in the SAME
    layer as the installs, in both the main RUN and the studio RUN. npm 11
    caches every platform tarball it downloads, including the ones the prune
    then deletes, so the cache grew faster than the tree. Nothing at runtime
    reads it: build is root, container is developer with its own cache in $HOME.
  - prune_foreign_esbuild -> prune_foreign_natives: now covers both MEASURED
    families. Clipboard keeps linux-$arch-gnu AND -musl because its napi-rs
    loader picks between them at runtime via its own isMusl() probe; the musl
    package is a 420-byte stub. The bare @mariozechner/clipboard wrapper has no
    hyphen suffix and cannot match the pattern.

Verified on arm64 before writing the glob — a widened rm -rf against a tree you
cannot inspect is the one change shape not to write blind, which is why the
order was update-then-patch. Exercised against a copy of the real trees with
foreign dirs fabricated back in (aix-ppc64, android-arm64, darwin-arm64,
win32-x64, linux-x64): all removed, host linux-arm64 kept at both sites, 21 MB
freed, require('@mariozechner/clipboard') still loads and exports all 18
functions, esbuild.transformSync still compiles TS at both sites. v1.9.1's own
arm64 validation also passed here; CI could only smoke amd64.

Two sentinel assertions, because the size gate did not catch this: it has
~225 MB of margin, so 131 MB of residue stayed green. Both were verified RED
against the running v1.9.1 image and GREEN against a pruned tree. The cache one
refuses to run as non-root: test ! -d /root/.npm on mode-700 /root would
otherwise pass for the wrong reason. Size-failure diagnostics now list cache
paths too — they previously enumerated only node_modules and /opt, where these
bytes were not.

Also fixed: -printf '%f\\n' reaches the shell with both backslashes (confirmed
from the published image's recorded created_by), so v1.9.1's progress line
printed a mangled "li ux-arm64" — find emitted a literal backslash and tr ate
the n out of the name. Single backslash now.

Deliberately not purged: /tmp/node-compile-cache (1.3 MB). The manifest RUN
calls pi --version again, so deleting it earlier only relocates those bytes into
that layer — today's manifest layer is 128 kB precisely because it finds the
cache warm.

Dockerfile.base untouched, so no base rebuild: this rides the next release.

AGENTS.md

@@ -280,8 +280,10 @@ shipped the same image bytes); preventatively fixed for `PI_VERSION` +
 image. Verifies binaries, repo clones, runtime deployment (waits for
 keybindings + mempalace bridge + ≥4 extensions before sampling — fixes
 the parallel-build-load race documented in opencode-devbox c6f9d11
-2026-06-08), and image size threshold (3500 MB; revisit after a few
-releases as actuals settle).
+2026-06-08), build-time leftovers (see below), and image size threshold
+(3800 MB in `SIZE_THRESHOLD_MB`; revisit after a few releases as actuals
+settle — this doc said 3500 until 2026-09-11, after the bar had already
+moved twice).
 
 If smoke fails on size threshold but build is otherwise fine: bump
 `SIZE_THRESHOLD_MB` in scripts/smoke-test.sh in a follow-up commit and
@@ -289,6 +291,17 @@ re-run. The threshold exists to catch *runaway* growth (an accidental
 texlive bake-in, a forgotten chrome dependency), not to block ordinary
 upstream bumps.
 
+**The size gate is not a substitute for naming the residue.** It carries
+~225 MB of deliberate margin, so v1.9.1 shipped +131 MB of pure build
+residue — 110 MB of it npm's own download cache under `/root/.npm`, the
+rest foreign platform packages — and stayed green. Two named assertions
+now cover exactly that: no foreign npm-11 platform packages beyond the
+host arch (`@esbuild/*`, `@mariozechner/clipboard-*`), and no
+`/root/.npm` in the image. Note the second one refuses to run as
+non-root: `test ! -d /root/.npm` on mode-700 `/root` would otherwise
+pass for the wrong reason, which is the failure shape to watch for in
+any assertion about a path you may not be able to read.
+
 ## Build pipeline notes
 
 - **Two-phase**: base + variant. Base is rebuilt only when