API freeze for 1.0: lock index.api.md, strict semver

[nam-hle]

Lock the public API surface (index.api.md), commit to strict semver, document stability guarantees. Gate for 1.0.

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[nam-hle]

Deferring. Too early to freeze the public API — the surface is still moving (keyed register API just landed in #688, group/namespace ergonomics #691 and type-safe dependsOn #690 still open, all of which touch the public types). Freezing now would lock in shapes we still want to change. Revisit once those land and the API has settled.