refactor(generation): add fixed stage runner

[lpukatch]

Packet 3

Retargets the fixed generation stage runner to dev.

Changes

• Add deterministic GenerationStage and StageRunner contracts.
• Replace the inline full-generation phase sequence with private stage functions and a fixed static stage list.
• Preserve bootstrap, terminal handling, processing-run ownership, generation-lock lifecycle, progress, cancellation, and timing semantics.

Scope

• No plugin runtime, stage discovery, retries, dynamic registration, parallelization, or caller migration.
• The separate date-fixture stabilization is already present on dev.

Verification

• Full backend suite: 3,478 passed.
• Ruff and Pyright passed.
• Frontend build passed.