measure(explore): the factory-closure envelope premise does not hold (CG-27)

CG-27 asked whether the >50%-of-file envelope drop should cover `function` /
`method`, so a `createFoo()` factory returning an object of closures stops
merging every closure inside it into one cluster. Measured on a hermetic
fixture, it should not, and the issue is closed as obsolete with CG-30 credited.

Two mechanisms already absorb the shape. shrinkCluster orders members by
(importance desc, size ASC) and refuses any member that overruns the cap once
something is kept, so a file-spanning member is only selected when it is the
sole member of the top importance tier — eight of nine query shapes never
selected it at all. When it IS selected, CG-30 windows it on whole lines, so
the file still delivers bounded, readable source (6 of 9 closure definitions
in that configuration).

Dropping the range instead SPLITS the file, and only the first-chosen cluster
may be shrunk: a trivial 7-line cluster won the density tiebreak and the
answer-bearing cluster was dropped whole — rank-#1 file 7,539 chars and 7 of 11
closures to 397 and none. Reaching the same intent more carefully (defer the
envelope MEMBER inside shrinkCluster, leaving clustering untouched) is noise:
69 vs 68 closure definitions across nine query shapes. Nothing shipped.

Adds the fixture, the probe, a standing gate on the outcome, and the record —
including a real defect the measurement exposed on the epic tip: django's
query.py leaves 8,212 of 10,135 unspent and drops a score-290 cluster to keep a
score-14 one. Filed separately.

No behaviour change, so no CHANGELOG entry.

scripts/agent-eval/probe-factory-closure.mjs

@@ -26,7 +26,10 @@ import { fileURLToPath, pathToFileURL } from 'node:url';
 const HERE = dirname(fileURLToPath(import.meta.url));
 const REPO_ROOT = resolve(HERE, '../..');
 const FIXTURE = join(REPO_ROOT, '__tests__/fixtures/factory-closure-ts');
-const TARGET = 'src/stores/dashboard-store.ts';
+const targetAt = process.argv.indexOf('--target');
+const TARGET = targetAt >= 0 ? process.argv[targetAt + 1] : 'src/stores/dashboard-store.ts';
+const factoryAt = process.argv.indexOf('--factory');
+const FACTORY = factoryAt >= 0 ? process.argv[factoryAt + 1] : 'createDashboardStore';
 
 const argv = process.argv.slice(2);
 const asJson = argv.includes('--json');
@@ -60,10 +63,10 @@ try {
   // Inner function definitions, straight from the index — the symbols the file's
   // enclosing factory range would otherwise swallow.
   const nodes = cg.getNodesInFile(TARGET);
-  const factory = nodes.find((n) => n.name === 'createDashboardStore');
+  const factory = nodes.find((n) => n.name === FACTORY);
   const inner = nodes
     .filter((n) => (n.kind === 'function' || n.kind === 'method')
-      && n.name !== 'createDashboardStore'
+      && n.name !== FACTORY
       && factory && n.startLine > factory.startLine && n.endLine <= factory.endLine)
     .sort((a, b) => a.startLine - b.startLine);
   cg.close?.();