feat(scan): size portions from measured per-answer cost and isolate the kill culprit

The 7.8 trace showed the completion loss was a sizer death spiral, not
expensive families: after two kills the budget fell to 3 sink visits and
never recovered, because portions finishing in 25-30 s sat under an assumed
70 s fixed start and read as "no information", so the budget could only
shrink.

- taint.sc streams each answer's own cost (`ms`); the backend carries it as a
  reserved `__timing__` list through shards, partitions and reuse.
- The sizer budgets predicted seconds from per-family seconds-per-visit rates
  learned from those costs, and measures the fixed start as the wall time the
  answers do not explain (median), instead of assuming it.
- A kill names its culprit: the first unanswered id in request order. The ids
  behind it never started and are re-queued at once; the culprit is asked
  alone after the rest. One that still fails alone after more than a
  portion's target is reported as `query_too_expensive`, apart from
  `query_failed`.
- Portions grow id by id from the queue head, and a single-portion scan now
  goes through the queue too, so a kill there re-asks its remainder.
- Fix: the reuse wrapper wrapped only `run_batch`, so since 7.8 every
  portioned query bypassed the graph-reuse cache and its source-root
  normalization. `run_batch_once` is wrapped too.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01EmM1j46qgKMh3zqD1QMwEo

tests/test_cpg_queries_compile.py

@@ -621,3 +621,8 @@ def test_the_taint_query_streams_one_line_per_request(tmp_path: Path) -> None:
     assert json.loads(lines[0]).keys() == {"__census__"}
     assembled = extract_payload(done.stdout)
     assert assembled is not None and set(assembled) >= {"0", "1", "__census__"} and "__partial__" not in assembled
+    # Every answer reports its own cost, which is what the portioner sizes from and how a kill names the
+    # request that was running. Absent, the sizer falls back to guessing from wall time.
+    answers = [json.loads(line) for line in lines[1:]]
+    assert all(isinstance(a.get("ms"), (int, float)) and a["ms"] >= 0 for a in answers), answers
+    assert [t["id"] for t in assembled["__timing__"]] == ["0", "1"]