Harden addon test and release gates

[nicodes]

Scope

Gate layer for fieldsofrevik#149. This PR does not close the cross-repository issue; the behavior and release layers remain.

Self-verification evidence (Team No Review)

• Correction (fieldsofrevik#149): the initially published abbreviated source expanded to the wrong guessed SHA. That initial head was 590e53d0869319f6e20dc878c851032aa43f9dab; after the exact-head checkout fix, the current verified head is 4120a2915404fd0b56c229c526d36e8437701562.
• Source: aviorstudio/gd-pocketbase, studio-engineering/149-gd-pocketbase-gates, 4120a2915404fd0b56c229c526d36e8437701562.
• Baseline/control: remote main 7f5e98ffd305cb9b4c2bab356ca66364836cba7a; its tests pass on Godot 4.7.2 but the runner accepted zero-exit error logs and had no reach sentinel/package lifecycle gate.
• Change: exact Godot 4.7.2 SHA-256 install, fail-closed bounded runner, exact-head PR checkout, release ZIP closed manifest, tested-byte handoff, full-SHA publication actions, clean packaged editor lifecycle. No runtime behavior change.
• Versions: exact-head Actions runner Ubuntu 24.04.5 / image 20260907.300.1; Godot 4.7.2.stable.official.ed1daf0bf; engine archive SHA-256 cadd3204e728a35d3f13adb7fd0d7902636b79f6b95c40c265eb73b6c35329e4.
• Commands: local GODOT_BIN=/tmp/opencode/godot-4.7.2/Godot_v4.7.2-stable_linux.x86_64 ./tests/test.sh; ./scripts/package_addon.sh; ./scripts/verify_package.sh; GODOT_BIN=... ./scripts/verify_installed_package.sh. CI ran the same composite action at the head SHA.
• Negative/restored/reachable controls: success fixture passed; push_error+zero exit, overwritten quit(1), parse failure, 1 s timeout, unreachable assertion, unexpected error log, and missing test were each rejected. Four restored suites emitted assertion sentinels (4/2/5/4).
• Exact CI-assembled ZIP: SHA-256 be105c5336c101f1868ca7b7b7488cc0858ebb0bb4668af80e2b68171e9b3f0c; closed manifest 12 entries; no symlinks/traversal/development files.
• Installed bytes: tree digest e359ef4d3acd0d322fe5a81ee6094032f8faec02f1368c9101569b4f7e825b6f; clean editor → enable → restart → packaged smoke (2 assertions) → disable → restart passed; no owned autoload/config remained.
• Action identities: gdam-actions v0.0.2 verified commit d735444eb470194585def44521d5d91df2260e63; upload/download-artifact v4 full SHAs; CI read-only, credentialed publish isolated.
• README correction: labelled Correction cites originating issue #149.
• Browser evidence: N/A for this gate-only PR; no browser behavior changed. Web boundary runtime evidence belongs to the behavior layer.
• Exact-head CI: PASS — run 34717439113, checkout log confirms HEAD is now at 4120a29; job ci completed successfully.
• Untested/residual: release/GDAM publication is intentionally deferred until behavior layer. Board writes are temporarily blocked by GitHub GraphQL quota after membership readback.