docs: commit trailers close the traceability chain

Commits that implement, change, or withdraw a requirement carry a TRACES
trailer using the same token and syntax as the code tags, so one grep pattern
serves both.

This is the last link. Code tags say where a requirement lives; commit trailers
say when and why it changed, and `git log --grep=AR-012` then reconstructs a
requirement's whole history — which no other artifact provides.

A commit serving no requirement omits the trailer: absence is meaningful, and
inventing a tag to satisfy the form is how orphan tags get created. Withdrawing
a requirement counts as changing it, so the withdrawal stays findable.

Also updates the chain diagram, which still referenced the retired @implements
tag and the thematic A1..E8 IDs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CLAUDE.md

@@ -94,6 +94,32 @@ is what creates orphan tags.
 When adding a requirement, give it a parent and a register row. When adding code,
 give it a tag.
 
+### Commits name their requirements too
+
+Every commit that implements, changes, or withdraws a requirement carries a
+`TRACES:` trailer — the same token and syntax as the code tags, so one grep
+pattern serves both:
+
+```
+feat(audio): v1 content-derived audio signature
+
+Implements the §3 construction with the six previously-undefined
+parameters pinned, plus a golden fixture the plugin can be written from.
+
+TRACES: IR-004, IR-005, IR-007, IR-008 | SR-003
+```
+
+This closes the last link in the chain. Code tags say *where* a requirement
+lives; commit trailers say *when and why it changed* — `git log --grep=AR-012`
+then reconstructs a requirement's entire history, which no other artifact gives
+you.
+
+- Trailer last, after any `Co-Authored-By`.
+- A commit serving no requirement (formatting, tooling, a typo) simply omits it —
+  absence is meaningful, so do not invent a tag to satisfy the form.
+- Withdrawing a requirement is a change to it: name it, so the withdrawal is
+  findable later.
+
 - **Specs are requirements + current-state deltas.** Each requirement carries
   `Current:` and `Gap:` so the document doubles as a work list. Keep that shape
   when editing.

SPEC.md

@@ -334,12 +334,18 @@ unimplemented, and both are worth knowing.
 ### The chain
 
 ```
-PR-n   project requirement      (this document, §1)   — why the system exists
- └─ SR-n   system requirement   (this document, §3)   — what spans components
-     └─ A1…E8 / §n  software requirement  (component specs)  — what one repo does
-         └─ @implements tag     (source)              — where it actually is
+PR-nnn   project requirement    (this document, §1)   — why the system exists
+ └─ SR-nnn   system requirement (this document, §3)   — what spans components
+     └─ component requirement   (component registers)  — what one repo does
+         ├─ TRACES tag          (source)               — where it lives
+         └─ TRACES trailer      (commit message)       — when and why it changed
 ```
 
+The commit trailer is the last link and the only one that carries *history*:
+`git log --grep=AR-012` reconstructs everything that ever happened to a
+requirement, which no other artifact provides. Same token and syntax as the code
+tag, so one grep pattern serves both.
+
 ### ID scheme
 
 Zero-padded three digits throughout. **IDs are permanent**: a withdrawn