M3 Tauri vault filesystem and watcher

[h4yfans]

Summary

• Adds the M3 Tauri vault backend: guarded path resolution, atomic file IO, frontmatter parsing/serialization, note read/write/delete flows, vault preferences, vault registry, runtime state, and filesystem watcher support.
• Wires the new Tauri command surface for vault, shell, dialog, app state, protocol handling, capabilities, and memry-file:// renderer support.
• Adds Rust coverage for vault path safety, file IO, frontmatter, notes IO, preferences, registry, watcher behavior, plus drag/drop smoke docs and M4 crypto/keychain/auth planning prompts.

Validation

• pnpm --filter @memry/desktop-tauri typecheck
• pnpm --filter @memry/desktop-tauri capability:check
• pnpm --filter @memry/desktop-tauri cargo:test
• pnpm --filter @memry/desktop-tauri lint (0 errors; warnings emitted by the current Tauri lint baseline)