measure: claim 24 unflooring is +4 (NOT-SHOWN); asymmetric embedding prefixes are -5 (discarded)

Measured on the 75-query extended gold set (iteration 8's held-out extension)
against the certified stack baseline results-stack-ext.json, on the embedded
corpus. Three runs of the candidate, zero drift.

A. CLAIM 24 WITHOUT THE THRESHOLD - net +4, NOT-SHOWN, kept in the tree.
   fixed  : q14, q25 (in-sample paraphrase), q43, q52, q63, q67 (held-out)
   broken : q15 (paraphrase), q28 (associative)
   8 discordant, McNemar exact p = 0.2891, floor is 6.
   heldout_paraphrase 16.7% -> 30.0%, paraphrase 61.5% -> 69.2%.
   Every regression guard held: exact_rare 6/6, phrase 7/7, nonsense 10/10,
   superseded 2/3. Latency FLAT: p50 641 -> 632 ms.
   The in-sample half (+q14 +q25 -q15 -q28 = 0) was already on record in
   iteration 7's cmp-nogate.json, so only the held-out +4 is new.

B. ASYMMETRIC TASK PREFIXES ON THE EMBEDDER - net -5, REVERTED in this commit.
   Rationale was sound and the prediction was wrong, which is why it was worth
   measuring: nomic-embed-text is an asymmetric retrieval encoder and this file
   embedded query and document bare on both sides. Prefixing does exactly what
   the model card implies for the far-away cases - it rescued q42 (gold at
   GLOBAL COSINE RANK 25,564) and q39 - but it re-ranks the whole space and
   broke more than it fixed:
   fixed  : q24, q39, q42
   broken : q18, q19, q22, q31, q43, q44, q52, q63
   heldout_paraphrase 30.0% -> 23.3%, paraphrase 69.2% -> 53.8%.
   The corpus and the reproducer are kept (embed-corpus-prefixed.py,
   snapshot-pre-repair-20260806-embedded-prefixed.json) so nobody re-runs it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tools/retrieval-eval/comparison-stack-vs-unfloor.json

@@ -1,6 +1,6 @@
 {
  "baseline": "stack-ext",
- "candidate": "unfloor-ext",
+ "candidate": "unfloor-clean",
  "n_shared_queries": 75,
  "fixed_by_candidate": [
   "q14",
@@ -96,9 +96,9 @@
   "mrr@10": 0.32437728937728944,
   "nonsense_clean": "10/10",
   "superseded_outranks": "2/3",
-  "latency_ms_p50": 993.0,
-  "latency_ms_p95": 1544.7,
-  "latency_ms_max": 1809.0,
+  "latency_ms_p50": 632.5,
+  "latency_ms_p95": 992.5,
+  "latency_ms_max": 1177.8,
   "errors": 0,
   "by_category": {
    "associative": {