feat: Phase U U4 pipeline phase timeline report (PTR-) -- ordered schema event sequence per pipeline run

[cschanhniem]

Summary

• Adds PTR- (Pipeline Phase Timeline Report) schema: ordered event sequence for a full pipeline run
• VALID_PIPELINE_SCHEMA_TYPES (16): all schema types from Phases S+T+U (BSP through CSV)
• VALID_TIMELINE_VERDICTS: complete/in_progress/empty; complete requires both SAT+PCC present
• TERMINAL_SCHEMA_TYPES: SAT+PCC
• build() sorts events by created_at, auto-assigns phase (nomination/screening/ranking/evidence/integration)
• TimelineEvent: schema_type, artifact_id, phase, created_at
• dry_lab_only=True enforced; PTR- prefix validated
• 50 tests; BASELINE 10017→10080

Test plan

• 50 unit tests pass: constants, build happy paths, validate rejections, format output
• Regression test passes
• Smoke test: BSP+SAT+PCC → complete; BSP+RSR → in_progress; empty → empty