[Language] Add a code-level construct stability registry

[cssbruno]

Follow-up from #298 and PR #300.

Problem

#298 published the per-construct stability table (docs/language/stability.md)
and a guard test that checks every metadata keyword and supported g: directive
is mentioned in the doc. The tiers themselves (stable/partial/planned/
deprecated) live only in prose, so nothing asserts a construct's tier is correct,
and top-level blocks and endpoint forms are not cross-checked. The diagnostics
registry already models Stability in code (internal/diagnostics/registry.go);
the language constructs do not.

Relevant code:

• docs/language/stability.md: the prose table
• internal/lang/stability_doc_test.go: presence-only guard
• internal/diagnostics/registry.go: the Stability precedent
• lang.MetadataKeywords, view.SupportedDirectiveNames

Scope

Add a code-level construct stability registry (construct name -> tier), covering
blocks, metadata keywords, g: directives, and endpoint forms. Generate or
verify docs/language/stability.md from it so the doc cannot disagree with the
registry, and expose tiers through tooling (for example gowdk explain or a
docs command). Extend the guard to assert tiers, not just presence.

Acceptance Criteria

• A code registry assigns a tier to every block, keyword, directive, and
  endpoint form.
• The published table is verified against (or generated from) the registry.
• Planned/deprecated constructs link to the diagnostic code emitted on use.
• A test fails when a registry construct is missing from the table or its tier
  disagrees.

Verification

go test ./internal/lang ./internal/diagnostics ./internal/view

[cssbruno]

Implemented and merged in #307.