fix(#4130): parse phase-prefixed decision IDs (D4-01) (#4357)

* test(#4130): failing-first regression for phase-prefixed decision IDs

Add the #4130 matrix: D4-01/D12-01 across all three bullet forms, tags,
discretion, wrapped lead-ins, gate-level plan/verify end-to-end rows, and
parity properties (well-formed digit-prefixed ids parse to their exact id;
a non-digit injected into the prefix fails loud). Update the #2347
non-D-prefix fixture from D5-NN (now a legal grammar) to DEC-NN, and
graduate the representative d5-prefix corpus fixture from could-not-parse
to parsed-but-uncovered.

All new rows are RED against origin/next; they go green with the parser
fix in the next commit.

* fix(#4130): parse phase-prefixed decision IDs (D4-01)

The three declaration grammars, the parse-miss guard, the #3939 join
regexes, and the token evidence all anchored on the literal 'D-' (or
'**D-'), so an ID carrying a digit-run phase prefix between the leading
letter and the hyphen matched nothing — while the #2347 shape detector
correctly called those bullets decision-shaped, collapsing the whole
CONTEXT.md to could-not-parse with 0 extracted instead of a coverage
verdict.

Derive the extractor ID grammar from one shared DECISION_ID_SOURCE
('D[0-9]*-' + the existing alnum tail, full id captured), widen the
guard/join anchors to ID_ATTEMPT_SOURCE (bare 'D-' or a digit-initial
prefix run, so a typo'd 'D4x-01' fails loud while letter-initial prose
like 'Deferred-until' stays none-present), and align the bare-token
evidence. Both gates and the gap-checker share the parser, so all three
surfaces read phase-prefixed decisions now; the gate messages name the
accepted forms including the phase-prefixed one.

* docs(#4130): document the phase-prefixed decision identifier form

The canonical CONTEXT.md reference said decisions carry 'a sequential
D-NN identifier' with no mention of the optional phase-number prefix the
parser now accepts (D4-01) or the alphanumeric tail it always accepted
(D-INFRA-01). Name both in the Decision identifier format section, EN
and ja-JP.

* chore(#4130): changeset

* chore(#4130): backfill PR number in changeset

---------

Co-authored-by: sim <sim@local>

src/check-command-router.cts

@@ -329,12 +329,15 @@ function cmdDecisionCoveragePlan(projectDir: string, args: string[], raw: boolea
       uncovered: [],
       message: partialParse
         ? 'Decision coverage gate: decisions could not be fully parsed — one or more ' +
-          '`- **D-NN ...**` bullets appear malformed (missing `:` or ` — ` separator). ' +
-          'Fix the bullet format so all D-NN decisions can be read before re-running the gate.'
+          '`- **D-NN ...**` bullets appear malformed (missing `:` or ` — ` separator, or a phase ' +
+          'prefix that is not a digit run, e.g. `D4x-01`). Fix the bullet format so all decisions ' +
+          'can be read before re-running the gate.'
         : 'Decision coverage gate: could not parse decisions — possible format mismatch. ' +
           'The CONTEXT.md appears to be decision-shaped (has a <decisions> block, a decisions heading, ' +
-          'or D- tokens) but no D-NN bullets could be extracted. Check the formatting of the decisions ' +
-          'block and ensure bullets follow the `- **D-NN:** text` or `- **D-NN — title** body` form.',
+          'or D- tokens) but no decision bullets could be extracted. Check the formatting of the decisions ' +
+          'block and ensure bullets follow the `- **D-NN:** text`, `- **D4-NN:** text` (phase-prefixed), ' +
+          'or `- **D-NN — title** body` form. An ID grammar the parser does not support (e.g. `DEC-01`) ' +
+          'also lands here.',
     }, raw, undefined);
     return;
   }
@@ -433,9 +436,11 @@ function cmdDecisionCoverageVerify(projectDir: string, args: string[], raw: bool
       not_honored: [],
       message: partialParse
         ? 'Decision coverage verify (warning): decisions could not be fully parsed — one or more ' +
-          '`- **D-NN ...**` bullets appear malformed. Fix the bullet format in the CONTEXT.md decisions block.'
+          '`- **D-NN ...**` bullets appear malformed (missing `:` or ` — ` separator, or a phase ' +
+          'prefix that is not a digit run). Fix the bullet format in the CONTEXT.md decisions block.'
         : 'Decision coverage verify (warning): could not parse decisions — possible format mismatch. ' +
-          'Check the formatting of the CONTEXT.md decisions block.',
+          'Check the formatting of the CONTEXT.md decisions block (accepted forms: `- **D-NN:** text`, ' +
+          '`- **D4-NN:** text` (phase-prefixed), `- **D-NN — title** body`).',
     }, raw, undefined);
     return;
   }