test(gates)!: the size/complexity ratchet leaves the gate; audit_metrics reports it

sourceSize.contract.test.ts, its test kit and backend/tests/test_source_size.py
held 13 FE / 9 BE file ceilings and 64 FE / 32 BE function ceilings. No real
hit (E34 S2/S4): its history is its own birth (751e1ae5), two rounds of
fixes to its own logic (e432dc27, d1df76e8), the snapshot move (182ec311)
and ceiling edits inside feature fixes (5806adda, df4a79a1, and the S2/S3
cuts here). It caught no defect, went red 8 times on its own limits and drew
5 review findings about itself. The numbers stay visible: audit_metrics
reports file size and complexity, and an epic compares before and after.
docs/gates.md: the two size rows go; the C901 row says where it is read now.

frontend/src/lib/gates.snapshot.json

@@ -1505,7 +1505,6 @@
         "src/lib/saveLanguage.dom.test.ts",
         "src/lib/savedViews.test.ts",
         "src/lib/sortableA11y.contract.test.ts",
-        "src/lib/sourceSize.contract.test.ts",
         "src/lib/staleBuild.dom.test.ts",
         "src/lib/store.dom.test.tsx",
         "src/lib/subscriptionCache.test.ts",
@@ -1948,7 +1947,6 @@
         "src/lib/savedViews.ts",
         "src/lib/seekableEnd.ts",
         "src/lib/sortOptions.ts",
-        "src/lib/sourceSize.testkit.ts",
         "src/lib/staleBuild.ts",
         "src/lib/storage.ts",
         "src/lib/store.ts",