fix: use `declName?` pattern for `normalizeInstance` meta marking

The previous fix (#13043) used `isMetaSection` to determine whether
`normalizeInstance` aux defs should be marked `meta`. This caused
`deriving` in meta sections to fail: the deriving handler doesn't mark
the instance itself as meta, so the non-meta instance couldn't access
its meta-marked aux defs.

Switch to the existing `declName?` pattern (already used by `unsafe`
and `private_decl%`), which inherits meta status from the parent
declaration rather than the scope. This correctly handles both cases:
- `inferInstanceAs`: parent declaration is marked meta by
  `processHeaders`, so aux defs are also marked meta
- `deriving`: `declName?` reflects the actual meta status of the
  declaration being elaborated, avoiding the mismatch

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>