corpus: align determinability key with the schema column

The truth files used `determinableFromResponseDocument`; the schema column
and gateEvaluableFromResponse() use `determinableFromResponse`. A silent
name drift means the flag reads as its default (true), and R-B6C — "two bid
copies submitted", which no response document can ever establish — starts
failing bidders on a packaging detail.

Renamed in rfp.truth.json and validate-responses.py, and added the mirror
check to validate-truth.py so the drift cannot recur unnoticed:

  gates 13, evaluable from the response 12, excluded 1
    excluded: R-B6C  Two bid copies submitted.

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

corpus/friendship-pcs/rfp.truth.json

@@ -241,7 +241,7 @@
       "page": 5,
       "text": "Two bid copies submitted.",
       "criterionIds": [],
-      "determinableFromResponseDocument": false,
+      "determinableFromResponse": false,
       "$comment": "A physical submission fact, not a document-content fact. No extraction of any response can establish it, and no better parsing would help. Kept as a mandatory gate because the SFA does enforce it — but it must be satisfied out of band. The schema has no concept for this today; see the note raised on #10."
     },
     {