feat/phase3-perf-and-refactor

[TwDover]

Phase 3: parallel WAV export, route→services split, deprecation/logging cleanup

Performance & refactor pass on Phase 3 of the roadmap.

WAV export (the real "1–2 min" wait — generation is only ~0.5s):

• offlineRender now renders each part on its own OfflineAudioContext
  concurrently (Chromium runs them on separate threads — ~2.48x on 8 cores),
  sums them, and applies the master limiter once in a raw-WaveShaper pass that
  reproduces makeMasterLimiter bit-for-bit. Output is mathematically identical
  to the old single-pass mix. New offlineMix.test.ts covers the sum logic.
• Auto-pause live playback while a WAV export runs: the render walks the
  timeline on the main thread and starved the Transport scheduler, freezing
  playback mid-export. MIDI export was never affected.

Route god-files split into services/ (handlers now thin, verified by 135 tests):

• generation core (_run_attempt, progression choice, style blend/groove
  overlay, voice-leading + quality helpers) -> services/generation.py
• song arrangement engine (_generate_song_sections + helpers, combined-MIDI
  writer, bridge-escape) -> services/song_builder.py
• routes_generate.py 1299->636, routes_song.py 1498->889; removes the
  route->route generation import (both files now depend on the service).

Cleanups:

• Deprecation warnings: test dep httpx -> httpx2 (Starlette TestClient);
  Electron console-message rewritten for the 36+ single-details signature.
• Logged 6 genuine silent-fault except blocks (record_export_keep, malformed
  priors, style-load for track names, ...); by-design fallbacks left as-is.

Also retired the obsolete "parallelize section generation" roadmap item
(measurement showed generation is ~0.5s, not the bottleneck) and added a
Chromium-over-CDP render benchmark (frontend/scripts/bench_render.mjs).

Tests: backend 135 green + ruff clean; frontend 92->96 green + vue-tsc/eslint clean.