[B1] Freeze Beta-era public package surface and stability classes

[SisyphusZheng]

Required work

Classify every public export as: stable candidate / experimental / internal-importable / compatibility-only / deprecated. Do not expose future speculative APIs. Clarify canonical semantic owner even where physical source remains temporarily in another package; if App/Element physical contract migration is safe and cheap, do it, otherwise document semantic owner / physical source / compatibility reason / removal condition.

Non-goals

No new contracts/protocol package; no speculative API surface.

Acceptance

docs/current/PACKAGE_SURFACE.md carries the stability classification for every export and matches source mechanically.

Part of umbrella #1155. Depends on: Alpha.10 GO.

[SisyphusZheng]

Closed — B1.2 landed via PR #1266 (squash-merged to dev at 17ec6a57), CI green at merge (autoflow-ci + pr-full-ci-evidence pass).

Acceptance evidence:

• docs/current/PACKAGE_SURFACE.md now carries a stability classification (stable-candidate / experimental / internal-importable / compatibility-only / deprecated) for every named export of every published subpath of all five packages (~180 exports, 26 subpath entries), in a machine-readable package-export-classes block plus human-readable tables.
• Mechanical match enforced: deno task package-surface:check extended to enumerate actual exports via the same TS module resolution as the public-interface snapshot; unclassified/stale/unknown-class/prose-missing entries fail the gate (RED proven with three mutation tests; 6 new unit tests in tools/check-package-surface.test.ts).
• Semantic owner vs physical source documented for App/Element contract types sitting on ADR-0122 frozen files (migration evaluated, rejected as unsafe — documented with owner / physical source / compatibility reason / removal condition instead, per packet rule).
• Publishability: deno task pack:dry-run → 0, deno task package-artifacts:check → 0; no product-code edits required (zero export drift found); no relative cross-package imports introduced.
• Non-goals honored: no new contracts/protocol package, no speculative API surface, no ADR-0122 frozen paths touched.