tactic search: announce the learned ranking only when it reordered a ladder

[AkiraTamai]

Summary

Follow-up to #106. The run-level log line claimed a learned ranking whenever data/tactic_search_history.json was non-empty, even for runs where no obligation's ladder actually moved — the opposite of the per-atom tactic_search.history_ranked field, which #106 fixed to mean "this obligation was reordered" (spec §12.4).

The announcement is now emitted lazily, from the first obligation whose ladder the artifact actually reordered:

-if tactic_search_enabled and not tactic_search_history.is_empty:
-    print(f"tactic search ladder ranked by {args.tactic_search_history} (fingerprint …)")
 ...
 # in _run_tactic_search_stage, per result:
+if result.history_ranked and not any(o.history_ranked for o in results.values()):
+    print(f"tactic search ladder ranked by {history_path} (fingerprint {result.history_fingerprint})")

So it still prints at most once per run and carries the same path/fingerprint, but only when the ranking was applied. _run_tactic_search_stage gains a history_path parameter for the message (defaulting to HISTORY_PATH); no behavioural change to the search itself, and no contract, schema or artifact change.

test_ranking_is_announced_only_when_an_obligation_was_reordered pins both directions: a non-empty artifact scoped to a different (obligation_class, stage) prints nothing, an applicable one prints the line.

Verification

• PYTHONPATH=scripts MUMEI_LEAN_SKIP_LIVE=1 python -m pytest -q → 307 passed, 10 skipped
• PYTHONPATH=scripts PATH="$HOME/.elan/bin:$PATH" python -m pytest tests/test_lean_bridge_e2e.py tests/test_tactic_search.py tests/test_tactic_history.py -q (real lake build) → 44 passed

Link to Devin session: https://app.devin.ai/sessions/99669609dea14e2ba7a541689de09a0a
Requested by: @AkiraTamai

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.