Add packed public API snapshots and semver surface diffs

[woksin]

Problem

Components verifies that every public subpath loads, compiles under strict TypeScript, and carries TSDoc. Those gates prove validity, but they do not produce a compact reviewable structural diff of the exported API between releases. A semver label records intent rather than proving the actual public-shape delta.

Direction

Add generated, reviewable API snapshots derived from the real packed export map:

• one snapshot per typed public subpath or one deterministically partitioned package snapshot;
• include exported names, signatures, generics, members, deprecations, and release tags;
• exclude private/unreachable implementation exports;
• compare against the base release and classify additions/removals/narrowing;
• require explicit semver intent for breaking changes without replacing human API review.

The snapshot generator must consume packed declarations, not workspace source aliases.

Acceptance criteria

• All 24 typed JS subpaths are represented from the packed archive.
• Deterministic regeneration produces no churn.
• Planted removal, narrowing, and undocumented export changes fail.
• CI uploads or comments a readable base-vs-head API diff.
• The centralized semver-label policy consumes or references the structural result without duplicating organization policy.