Use NoInfer in GetTagMetadata to prevent inference widening

[tamish-max]

Closes #848

Summary

After auditing all exported types in source/, GetTagMetadata is the only type where NoInfer (TS 5.4+) is applicable. TagName appears in both the constraint (Tag<TagName, unknown>) and as a standalone parameter, creating competing inference sites. When Type carries multiple tags, TypeScript could widen TagName from the structural shape.

Change

 export type GetTagMetadata<
-  Type extends Tag<TagName, unknown>,
+  Type extends Tag<NoInfer<TagName>, unknown>,
   TagName extends PropertyKey
 > = Type[typeof tag][TagName];

Why only GetTagMetadata?

All other types were reviewed. NoInfer requires: (1) a type param in a constraint AND as a standalone param, (2) competing inference sites that can conflict. Most type-fest types use extends infer for computation captures or have single inference sites — neither pattern benefits from NoInfer.

Test

Added multi-tag test in test-d/opaque.ts verifying precise metadata recovery when a type carries multiple tags.

Note: Draft PR #1403 proposes the same change but lacks tests. This PR adds the regression test.