fix(app): turn the APK download back on for v0.7.0

platform.json's androidApk.serviceable goes to true. It was false because
v0.5.0's build did not work. v0.7.0's released APK (SHA256 matches
SHA256SUMS) was installed on an Android 16 emulator, connected to the
public demo and read its Rust servers, so /app/ links the download again.

The comment beside the flag now says when it was last checked and to set
it back to false when `asset` moves to a release nobody has run. PLAN.md
D29 gets a dated note.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E14m6SuuY6i1vASFeGDBeY

PLAN.md

@@ -956,6 +956,8 @@ deliberately so — no fetch can tell whether an APK runs. `checkFacts.mjs` asse
 still exist and that `minSdk` still says what "Android 10 or newer" claims, so the link is correct
 the moment a working build flips the boolean. The panel is not removed while the link is off: a page
 that omitted sideloading would read, to somebody who knows the APK exists, as a page hiding it.
+*(2026-10-31: the link is back on. Android `v0.7.0`'s APK was installed on an Android 16 emulator,
+connected to the public demo and read its Rust servers, so `serviceable` is `true`.)*
 
 **Three mechanisms this phase added that the plan did not anticipate.**