fix(#4424): assert the launcher snippet's env surface is covered by the scrub lists (#4503)

* chore(#4424): assert launcher snippet env surface is covered by scrub lists

SNIPPET_SCRUB is hand-maintained for vars TEST_ENV_BASE's registry-derived
list can't carry. Nothing asserted the union actually covers every
${VAR:-default} arm in _runtime-launcher.snippet.sh, so a new runtime-home
arm with no scrub entry could drift silently — the #4205 shape, one door
over. Adds (A2): extracts every ${[A-Z_]+:-} capture from the snippet and
checks membership in TEST_ENV_BASE, SNIPPET_SCRUB, or the two vars the
snippet/fixtures set themselves (RUNTIME_DIR, GSD_TOOLS).

* fix(#4424): allow digits in the (A2) fallback-var regex

CodeRabbit review on fork PR #37: [A-Z_]+ silently drops any \${VAR:-...}
capture whose name contains a digit (e.g. CLAUDE2_CONFIG_DIR) instead of
flagging it uncovered, defeating the guard's own purpose. Matches bash
identifier syntax instead: leading letter/underscore, then alnum/underscore.

* fix(#4424): rename SELF_ASSIGNED to reflect RUNTIME_DIR's real provenance

Gemini adversarial review (agy) on fork PR #37: RUNTIME_DIR is an external
input the snippet reads via \${RUNTIME_DIR:-...}, never assigns — every
fixture sets it in-script before sourcing the snippet. Only GSD_TOOLS is
truly snippet-self-assigned. SELF_ASSIGNED conflated the two; renamed to
CALLER_OR_SELF_ASSIGNED. No behavior change.

Reviewed and rejected: moving RUNTIME_DIR into SNIPPET_SCRUB (blanking it
is indistinguishable from unset to the resolver's own \${RUNTIME_DIR:-...}
fallback, re-opening the #4205 ambient-leak this suite guards against —
see the existing comment at line ~1801); widening the regex to mixed-case,
colon-less \${VAR-default}, or \${VAR:=default} forms (none exist in the
snippet, and the issue's own spec scopes this to \${[A-Z_]+:- captures);
stripping bash comments before matching (the snippet is one physical line
with zero '#' characters, so no comment can exist in it).

* fix(#4424): guard CALLER_OR_SELF_ASSIGNED against silent future additions

trek-e review on PR #4503: a future ${VAR:-default} arm could be dropped
into this set without confirming it is genuinely caller-supplied/
self-assigned rather than a real coverage gap. Adds a comment requiring
justification for any addition, pointing to SNIPPET_SCRUB as the default
when in doubt. No behavior change.

* fix(#4424): guard (A2) against a vacuous pass on empty extraction

agy adversarial review (gemini-3.8-flash-high, /gsd-review lane) on PR
#4503: if the snippet becomes unreadable/truncated/renamed, matchAll
yields zero matches, uncovered stays [], and assert.deepStrictEqual
passes vacuously — same "guards the guard" gap the sibling (E)-adjacent
tests already close with assert.ok(files.length > 0, ...). Asserts
extracted.length >= 15 before filtering.

Reviewer's second finding (regex misses colon-less ${VAR-default}) is not
applied: no such form exists in the snippet today, and 688cc1c already
recorded this exact widening as scope creep the issue's own spec (${[A-Z_]+:-)
does not ask for.

---------

Co-authored-by: Test <test@test.com>
Co-authored-by: Tom Boucher <trekkie@nomorestars.com>

tests/runtime-launcher-parity.test.cjs

@@ -380,6 +380,42 @@ describe('runtime-launcher-parity (#373)', () => {
     );
   });
 
+  // ─── (A2) Snippet's ${VAR:-default} surface is fully covered by the scrub lists (#4424) ──
+  // SNIPPET_SCRUB above is hand-maintained for vars TEST_ENV_BASE cannot derive.
+  // Nothing previously asserted the union actually covers every fallback arm in
+  // the snippet, so a new runtime-home arm with no scrub entry could drift
+  // silently — the same shape as #4205, arriving through the hand-listed half.
+  test('(A2) every ${VAR:-default} arm in the snippet is covered by TEST_ENV_BASE, SNIPPET_SCRUB, or a caller-supplied var', () => {
+    // RUNTIME_DIR: an external input the snippet reads, never assigns — every
+    // fixture that sources the snippet sets it in-script before doing so.
+    // GSD_TOOLS: the snippet assigns this one itself, before this arm's
+    // ${GSD_TOOLS:-} check runs.
+    // Any addition here must justify, in a comment like the two above, why the
+    // var is genuinely caller-supplied/self-assigned — not a real coverage gap
+    // silenced by exemption. When in doubt, add a SNIPPET_SCRUB entry instead.
+    const CALLER_OR_SELF_ASSIGNED = new Set(['RUNTIME_DIR', 'GSD_TOOLS']);
+    const snippetContent = fs.readFileSync(SNIPPET_FILE, 'utf8');
+    const covered = new Set([...Object.keys(TEST_ENV_BASE), ...Object.keys(SNIPPET_SCRUB), ...CALLER_OR_SELF_ASSIGNED]);
+    const extracted = [...new Set(
+      [...snippetContent.matchAll(/\$\{([A-Z_][A-Z0-9_]*):-/g)].map((m) => m[1]),
+    )];
+    // Guards the guard: a truncated/renamed/unreadable snippet would make
+    // `extracted` empty, and an empty `uncovered` below would pass vacuously.
+    assert.ok(
+      extracted.length >= 15,
+      `expected the snippet to yield many distinct \${VAR:-default} arms, got ${extracted.length}`,
+    );
+    const uncovered = extracted.filter((name) => !covered.has(name));
+
+    assert.deepStrictEqual(
+      uncovered,
+      [],
+      'Snippet fallback arm(s) not covered by TEST_ENV_BASE, SNIPPET_SCRUB, or a caller-supplied var — ' +
+        'add a SNIPPET_SCRUB entry (or confirm the capability registry should carry it):\n' +
+        uncovered.join('\n'),
+    );
+  });
+
   // ─── (B) Exactly ONE canonical preamble per using file ───────────────────
   test('(B) each workflow .md using gsd_run contains exactly ONE canonical preamble, before the first gsd_run call', () => {
     const preamble = expectedPreamble();