Prove native Tabs assembly, editing and frontend persistence

[noeltock]

Problem

Native Tabs were registered but lacked an explicit input contract and editor persistence proof. Closes #117.

Solution

Qualify the existing generic assembler without changing production code. The guide now supplies both native label locations and limits the claim to page content.

Behaviour	Read first	Proof
Complete labels, nesting, rich content and independent instances survive assembly	dev/test/intent-tabs.test.ts	Three headless variants pass
Frontend works before editor effects; native edits persist	scripts/proof-tabs-playwright.mjs	WordPress 7.1 browser run passes
Runtime proof stays outside normal unit tests	vitest.wordpress.config.ts	Explicit suite and existing CI artifact root

Diff

+551 −2 · 7 files · no production API change

 realize -> existing generic createBlock -> shared finalisation/gate
+ explicit Tabs fixture -> three headless round trips
+ frontend before editor -> save/reopen -> label/body edits -> save/reopen
+ two-instance ARIA/click/keyboard proof -> native WordPress control
+ ASSEMBLE.md: matching list/panel labels and page-content boundary

Testing & verification

Reviewed revision: c564240 · Node 22.23.1, WordPress 7.1, PHP 8.3.35, Chromium 149.0.7827.55.

• npm run verify passed: 709 tests passed, four existing skips; typecheck, build, corpus hashes and package checks passed.
• npx --no-install vitest run dev/test/intent-tabs.test.ts dev/test/intent-query.test.ts dev/test/intent.test.ts dev/test/engine-skill.test.ts dev/test/skill.test.ts passed: 23 tests. The final guide edit additionally passed both skill suites (eight tests).
• npx --no-install vitest run --config vitest.wordpress.config.ts dev/test/proof-tabs-wordpress.test.ts passed. Initial cold container startup timed out; the retained rerun passed after completing the same owned environment. Both save/reopen pairs had equal content hashes, clean reopened state and no invalid blocks. Arrow keys moved focus; Enter/Space activated tabs, matching the native control.
• Local receipt: /tmp/block-runner-tabs-evidence/tabs; CI retains the same evidence under its existing WordPress artifact root.
• git diff --check ff52333 passed.

Not verified: nested Tabs, registered-block label fields/overrides, model conversion quality, manual visual acceptance or a release. No model benchmark ran.

Risk / rollout

Production assembly is unchanged. The added browser proof requires the existing WordPress/Chromium prerequisites and retains failures instead of skipping them. The label rule follows Core metadata; keyboard assertions follow Core actions.

Detection: headless or WordPress CI fails on a persistence regression. Rollback: revert this PR; no migration or published package is involved.