fix: handle propositional and decidable instances in sym canonicalizer

This PR refactors instance canonicalization in the sym canonicalizer to
properly handle `Grind.nestedProof` and `Grind.nestedDecidable` markers.
Previously, the canonicalizer would report an issue when it failed to
resynthesize propositional instances that were provided by `grind`
itself or by the user via `haveI`. Now, resynthesis failure gracefully
falls back to the original instance in value positions, while remaining
strict inside types. This also removes the separate `cacheInsts` cache
in favor of the existing type/value caches via `withCaching`.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>