build: treat warnings as errors on compiler(>=6.4)

[grdsdev]

Summary

• Adopts the treatAllWarnings(as: .error) gate from pointfreeco/sqlite-data#467, active only when #if compiler(>=6.4).
• Stacked on build: enable ImmutableWeakCaptures and InferIsolatedConformances #1071.

⚠️ Known risk — please read before merging

This repo currently has pre-existing deprecation warnings that this setting will turn into build failures once the CI toolchain moves to Swift 6.4+ (e.g. defaultStorageEncoder/defaultStorageDecoder in Storage, deprecated APIs under Sources/Realtime/Deprecated/). I could not test the gate actually engaging locally — this environment's toolchain is Swift 6.3.3, so the #if compiler(>=6.4) branch never activates and this PR is a no-op today.

Before or alongside bumping CI to a 6.4+ toolchain, those pre-existing warnings should be resolved (or the affected targets excluded from this setting), otherwise the build will start failing with no code change once the compiler updates.

Test plan

• swift build succeeds (gate inactive on this toolchain, so unchanged from PR1)
• Re-verify once CI/local toolchain reaches Swift 6.4, since the gate can't be exercised here

[grdsdev]

Won't do for now — the pre-existing deprecation warnings in this repo would need to be resolved first, and this couldn't be validated against a real Swift 6.4 toolchain. Revisiting later if/when we bump CI to 6.4+.