advance: adopt declared target on first multiple_new_tests, no extra suite runs

[geuben]

Summary

• Adds _outcome_from_verdicts helper: looks up the already-executed test verdict from the Verdict objects returned by run_projects, returning FAILED/PASSED/None.
• Wires evaluation into the single-candidate adoption branch: instead of asking the agent to run tdd advance again, the adopted target is judged from the run that already happened — committing RED or demanding sensitivity in the same advance call.
• Adds _disambiguate: resolves an ambiguous multiple_new_tests situation by normalisation match (issue advance: normalise describe/test id separator so a formatting-only difference is not a declared_test_mismatch #57) or unique same-file candidate, without asking the agent; genuinely ambiguous cases still route through tdd target.
• Wires _disambiguate into the len(candidates) > 1 branch so same-file disambiguation also evaluates in one advance.
• Updates R8.9 in docs/PRD.md to document same-advance evaluation and the two disambiguation rules.

Quality gates

• uv run ruff check src tests — all checks passed
• uv run pytest — 424 passed (418 baseline + 6 new)

Test coverage note

All decision-point operators in new code are covered by at least one test (pseudo-mutation analysis passed).

Closes #72