release: date the v1.9.2 section for the tag

CI checks out the tag, so the CHANGELOG heading has to name the version
before the tag exists, not after. v1.9.1's tagged tree carried no
"## Unreleased" either -- same convention, made explicit here.

Contents of v1.9.2 (all measured, nothing inherited):
  - 1baba79  the +131 MB v1.9.1 residual: /root/.npm (110 MB) plus
             @mariozechner/clipboard-* foreign natives at BOTH install
             sites (global + /opt/pi-fork's nested pi-coding-agent copy)
  - 852f900  smoke sentinels that name the residue instead of letting
             the size gate's ~225 MB margin swallow it
  - dab989b  mempalace-toolkit: the owed-withdrawal suite gate
             (node --check never type-strips)
  - 9aaff26  vendored mempalace skill snapshot -> e9e45f7, which is what
             turns the currently-red canary green

Deliberately NOT in this tag: DOCKER_HUB.md's "~1.1 GB" size claim is
low (Hub says v1.9.1 = 1.37 GB, v1.8.14 = 1.23 GB). This release
*changes* the size, so no number is right both before and after it --
writing the predicted ~1.24 GB would be an unmeasured claim in a
user-visible page. Measure post-build, fix on the next tag, and extend
check-doc-drift.sh to gate size claims against Hub's full_size so the
number cannot rot silently again.

CHANGELOG.md

@@ -11,7 +11,7 @@ Pre-v1.0.0 tags followed the pi npm version (`v{pi_version}[letter]`).
 
 ---
 
-## Unreleased
+## v1.9.2 — 2026-09-14
 
 **The v1.9.1 residual is attributed and fixed: it was mostly npm's own download
 cache, not the platform binaries it looked like.** v1.9.1 pruned the 26 foreign