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>

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 **IDs are permanent**: a withdrawn