feat(live): determinate progress bars for preprocess & rank

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Summary

• Adds stacked progress bars to the --live panel for the preprocess and rank pipeline stages
• Bars fill incrementally as each stage executes, with a DONE! marker when both complete
• Extends SourcehuntStagePayload with an optional progress: float field (0.0–1.0)

preprocess: [████▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒]

→

preprocess: [████████████████████]
rank:       [████████▒▒▒▒▒▒▒▒▒▒▒▒]

→

preprocess: [████████████████████]
rank:       [████████████████████]
DONE!

Files changed

File	Change
event_payloads.py	progress field on SourcehuntStagePayload
runner.py	Thread progress kwarg through _emit_stage
preprocessor.py	Emit progress events at substep boundaries (static analysis → file enum → callgraph → reachability → semgrep → taint)
ranker.py	Emit progress per completed chunk
llm_activity.py	Subscribe to SOURCEHUNT_STAGE, track state, render bars in the Rich panel

Test plan

• ruff check passes on all modified files
• 181 related tests pass (-k "preprocess or ranker or llm_activity or event")
• Manual: run sourcehunt with --live on a small repo, confirm bars animate and show DONE!