perf: runtime performance pass, phase 1 (DB pragmas, batched scan writes)

Executes docs/superpowers/specs/2026-07-07-runtime-performance-pass-design.md:

- database.rs: every pooled connection now runs WAL + synchronous=NORMAL
  + busy_timeout=5000 + foreign_keys=ON via with_init (in-memory pools
  get the latter two). Fixes the SQLITE_BUSY failure mode under the
  parallel scanner and makes FK cascades work on every connection, not
  just whichever one last ran the old one-shot pragma in delete_folder.
- database.rs: prepare_cached for the per-video info_cache queries; new
  info_cache_put_many bulk upsert (single transaction) replaces the
  per-row autocommitted info_cache_put, with a round-trip unit test.
- library.rs: enrich_with_cache collects cache misses and flushes them
  once per channel; title sort uses sort_by_cached_key.
- app.rs: Title/ChannelAsc sorts use sort_by_cached_key instead of
  allocating two lowercased strings per comparison.
- Cargo.toml: panic=abort in release (crash-hook still fires; Cargo
  forces unwind for test harnesses). PACKAGING.md documents the local
  RUSTFLAGS target-cpu=native opt-in; repo stays portable.
- .gitignore: catacomb.db-wal / catacomb.db-shm sidecars.

Verified: 146 tests pass (integration suite exercises the pragmas
end-to-end), x86_64-pc-windows-gnu check green, live smoke shows
journal_mode=wal + batched cache writes landing correct rows.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/PACKAGING.md

@@ -12,6 +12,18 @@ scripts/package.sh appimage
 The script builds the release binary once and reuses it for every
 format. Output lands in `dist/` (gitignored).
 
+> **Building for your own machine?** Distributed packages must stay
+> portable, so the repo never sets `target-cpu`. If you compile locally
+> and only run the binary on the build host, you can opt into
+> host-specific SIMD (helps the ffmpeg-adjacent hot paths like the
+> perceptual-dedup hashing):
+>
+> ```sh
+> RUSTFLAGS="-C target-cpu=native" cargo build --release
+> ```
+>
+> Don't ship that binary — it will SIGILL on any older CPU.
+
 ## Linux formats
 
 ### .deb (Debian / Ubuntu / Mint)