feat(ranking): phase 1 -- tier-0 taint requests are no longer asked

A pair with no sink of its family in reach cannot yield a flow, in any
language under any dataflow model, so its taint request is never issued. The
count is recorded as requests_pruned -- exact, so not a degradation -- and a
test asserts that every other pair's request still goes out.

    637-file plugin, 500-region budget
    tier_counts   {0: 2161, 1: 60, 2: 20, 3: 211, 4: 48}
    pruned        2,161 of 2,500 = 86%        evidence pass 94.5s
    taint         339 requests -> 2,400s TIMEOUT

Gate met and exceeded; the scan still does not finish, and the reason corrects
phase 0. "1,148 ms per request" was an average over a 250-request set that was
88% tier 0 and answered instantly; the ~30 real requests in it cost ~9.5s each.
Pruning removes exactly the cheap questions and keeps the expensive ones, so it
cannot make them faster. The honest cost of a taint question that CAN have an
answer is 7-10s at callDepth=3, and that is what phase 2 and 5.13 design
against -- not 1.1s, and not 6.6s either. Both specs corrected.

869 tests pass.

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01T7XD7x5KonMdmGRkp63EQG

.kiro/specs/contributor-scan/tasks.md

@@ -826,8 +826,10 @@
     independent of the region's family, both repeated 2,500 times. Memoised across the batch (found while
     building flow-aware-ranking's evidence mode, which asks no dataflow and still took an hour), the same
     250 requests with both joins on: **2,026.3 s → 287.0 s, 8,105 → 1,148 ms per request, identical 5,779
-    rows.** A 7× speedup with nothing about the analysis changed. The base cost of a taint request is about
-    1.1 s, and the levers above still apply to it.
+    rows.** A 7× speedup with nothing about the analysis changed -- but "1.1 s" is an average over a set
+    that was 88% tier-0 pairs answering instantly. With those pruned (flow-aware-ranking phase 1), the 339
+    requests that CAN have an answer still exceed 2,400 s: **7–10 s per real request at `callDepth = 3`**.
+    The memoisation was real and stays; the per-request figure to design against is that one.
   - **The estimate this replaces was wrong and worth recording as wrong.** ">0.94s per request" came from
     assuming taint COMPLETED within its 2,400s ceiling. It timed out, so that was a floor presented as a
     figure. The measured cost is seven times higher.