fix: recover extension install collisions and release 0.3.9

[notmike101]

Summary

• recover catalog installs when an incompatible or otherwise untracked extension directory already occupies the destination
• preserve the displaced tree in .trash, fall back to a verified copy when Windows locks prevent rename, and restore the old tree if replacement fails
• serialize nested errors so extension scan/install failures retain their real messages and filesystem details
• bump host and SDK metadata to 0.3.9 and make the host-updater test fixture version-independent

Root cause

An incompatible zcode-tps 0.1.0 directory remained on disk after ZCode moved to 3.5.2. Startup correctly rejected it, but omitted it from the installed-record map. Catalog install therefore passed the in-memory duplicate check and collided with the hidden destination. Windows surfaced the collision as EPERM; the directory was also held open by the running Electron process, so a plain rename could not recover.

User impact

Catalog installs now self-heal stale hidden destinations without deleting the previous extension tree. Locked destinations use the same copy fallback already proven by extension updates, while retaining a rollback copy.

Validation

• bun scripts/package-release.ts --verify-only --tag v0.3.9
• bun run typecheck
• bun test (53 passed)
• clean detached Windows worktree: bun run check
• clean detached Windows worktree: npm pack ./sdk --dry-run
• clean detached Windows worktree: bun scripts/package-release.ts --tag v0.3.9
• live repair: verified and installed Token Speed 0.1.1 over the locked incompatible 0.1.0 tree, with 0.1.0 preserved in .trash