feat: add Unity asset export via optional agentdecompile[unity] extra

[bodencrouch]

Summary

Depends on #150 (targets that branch, not master, until it merges — this needed the [all] extra it introduces).

New unity-export CLI subcommand and src/agentdecompile_recovery/unity_assets.py, adapted from a bounded-memory UnityPy-based export script proven out on a large real-world Unity title where AssetRipper's whole-graph loading OOM'd on heavy Mesh/BlendShapeData.

• Exports textures, sprites, audio clips, text assets, and fonts one object at a time (freeing each immediately) — peak memory stays roughly constant regardless of asset file size on disk, unlike full project-reconstruction tools that load the entire asset graph before writing anything.
• Deliberately does not attempt meshes/materials/scene-prefab structure — that needs fuller project-graph tooling this module isn't trying to replace.
• UnityPy is optional (pip install agentdecompile[unity], or via [all]) and imported lazily — importing unity_assets never requires it installed; invoking the CLI command without it raises a clear ImportError naming the extra.

Also fixed (discovered running the suite on this branch)

• tests/test_rewrite_queue.py's two multiprocessing concurrency tests failed under Python 3.14 even on Linux (PicklingError on a locally-nested function target) — Python 3.14 changed the default multiprocessing start method away from fork on Linux too (previously only macOS defaulted away from it). Forced multiprocessing.get_context("fork") explicitly — same fix already shipped for the macOS-only case in fix: macOS multiprocessing failures + unused import in CI #152, now confirmed broader.
• source_parity_synthesize.py: removed an unused .state.now import (ruff F401) — pre-existing, unrelated, blocking lint CI.

Test plan

• 9 new tests for unity_assets.py (sanitization, file discovery, data-dir lookup, error paths, lazy-import failure message)
• 610 total unit tests pass
• ruff check clean
• Verified agentdecompile-recover unity-export --help and both [unity]/[all] extras resolve cleanly via uv pip install --dry-run

🤖 Generated with Claude Code

https://claude.ai/code/session_01Ros3797gzvmswnJudQ1Znk