Swift demangler upstream sync due

Upstream Swift moved under the vendored demangler since the last sync:

- Last synced ref: `swift-6.3.3-RELEASE` (2026-08-29)
- Newest upstream release tag: `swift-6.3.3-RELEASE`
- Commits touching `lib/Demangling` or `include/swift/Demangling` since the last sync:

```
dc98e9a Merge pull request #91728 from egorzhdan/egorzhdan/bitwise-copyable-friend
```

Run `scripts/sync-swift.sh` to refresh `vendor/swift` (records provenance in
`vendor/swift/SYNC.md` and runs the validation gauntlet), then review the
corpus snapshots before releasing.

This issue is opened monthly by the `swift-sync-reminder` workflow.

Start with scripts/sync-swift.sh and inspect how it refreshes vendor/swift, records provenance in vendor/swift/SYNC.md, and runs validation. Review the corpus snapshots afterward; done means the vendored sync, provenance record, validation gauntlet, and snapshots are consistent for the release.

Written by the indexing model from the issue text.