refactor(generation): add fixed stage runner

[lpukatch]

Packet 3

Implements Packet 3 of the generation modularization plan.

Changes

• Add deterministic internal GenerationStage and StageRunner contracts.
• Replace the inline full-generation phase sequence with private stage functions and a fixed static stage list.
• Keep bootstrap, terminal handling, processing-run ownership, and generation-lock lifecycle in generation.py.
• Preserve the existing team-channel stage alongside the documented pipeline stages.

Compatibility

• Preserves stage ordering, cancellation boundaries, timing attribution, raw progress events, API/SSE callback shape, and outer exception behavior.
• No plugin runtime, discovery, retries, dynamic registration, parallelization, or caller migration is included.

Verification

• Focused generation suite: 86 passed.
• Ruff passed.
• Pyright: 0 errors, 0 warnings.
• Frontend build passed.
• Full backend suite: 3,470 passed; 1 unrelated fixed-date failure in tests/matching/test_failure_taxonomy.py. Follow-up: teamarr-aj5c.

No later packet or plugin functionality is included.