docs: land the registers for schema v2 and the audio signature

Status and verification rows for the six requirements landed in this
branch, plus UT-029 … UT-052, and the generated traceability matrix that
`docs/traceability.md` holds in the other two components but was missing
here. `traces-report.json` is gitignored to match the server: the matrix
is committed, the JSON report is not, since nothing reads it back.

Two corrections rather than additions:

`AudioSignatureTests` was tagged UT-029 … UT-035, IDs the register had
already assigned to the schema tests. Renumbered to UT-038 … UT-044,
matching the register, which was right — duplicate IDs defeat the point
of IDs being permanent.

`ReadCappedAsync` implements JR-028's response cap and carried no tag,
so the requirement read as uncovered.

The gate reports 0 orphan tags.

TRACES: JR-002, JR-003, JR-028, JR-041, JR-042, JR-043, JR-044, JR-045 | SR-003

.gitignore

@@ -3,3 +3,12 @@ obj/
 .vs/
 .idea/
 artifacts
+
+# Python artefacts from the traceability tooling
+__pycache__/
+*.pyc
+
+# Generated traceability output — regenerate with the gate, never hand-edit.
+# `docs/traceability.md` is committed, as in the other two components; the JSON
+# report is not, since nothing reads it back.
+traces-report.json