Release prep v0.7.4: fix sys.path shim shadowing + changelog

[cryptopoly]

Summary

Two changes to unblock the v0.7.4 release:

1. Fix pytest tests/ collection error in tests/test_cache_strategies.py (ImportError: cannot import name '_find_pip_turboquant_path' from 'turboquant_mlx'). Root cause: ensure_extras_on_sys_path() was inserting the persisted extras dir at sys.path index 1, ahead of the repo root, which shadowed the repo-local turboquant_mlx adapter shim with the raw turboquant-mlx-full pip package staged into extras. The shim's exported helpers became unreachable. Fix: append extras to sys.path instead. Repo-local shims keep import authority across pytest, .venv dev, and Tauri-bundled launches.

   This was also a latent runtime bug — any user who clicked Setup → Install turboquant-mlx-full on the desktop app was running with the shim shadowed. The wrapped TurboQuant cache strategy still worked because cache_compression/turboquant.py calls into the upstream package directly, but the shim's _find_pip_turboquant_path / make_adaptive_cache / apply_patch adapter hooks were dead.

2. Backfill CHANGELOG.md for v0.7.2, v0.7.3, and v0.7.4. The changelog had jumped straight from v0.6.0 to the older v0.5.3 entry and was out of sync with the tag history. v0.7.4 entry groups the 90 staging commits into Cache strategies / CUDA quantization foundations / MLX video runtime / Preview UX / Speculative decoding / Windows stability / Studio polish themes.

Verification

• .venv/bin/python -m pytest tests/ -q → 1277 passed, 1 skipped
• ./scripts/pre-build-check.sh → 7 passed, 0 failed, 1 warning (turbo binary update WARN is informational)
• .venv/bin/python -c "from backend_service.app import create_app; import turboquant_mlx; print(turboquant_mlx.__file__)" → resolves to /Users/dan/ChaosEngineAI/turboquant_mlx/__init__.py (shim wins)

Test plan

• Python tests pass
• TypeScript tests pass (333/333 via existing pre-build script)
• npx tsc --noEmit clean
• Cache strategy validation passes
• Shim resolution preserved after ensure_extras_on_sys_path runs