fix(explore): damp ambient declaration files on flow queries (CG-28)

A file that declares nothing but types and that nothing in the index depends
on — a hand-written ambient `.d.ts` of global shims, vendored typings, module
augmentation — cannot answer a flow question: no bodies, no call edges, no
behaviour, nothing typed by it. But the identifiers it declares are exactly the
generic ones a prose question uses (`Body`, `Message`, `ImageMetadata`,
`ReadableStream`), so on term overlap it out-scored the implementation. Measured
on the new fixture: rank #1 and 51% of delivered source, with the flow's own
entry file pushed out of the response entirely.

Measured first, per the issue: the Wrangler `worker-configuration.d.ts` that
opened this is already handled by CG-25's banner detection, worth 15-46 points
of envelope share across four flow queries. CG-25 credited; only the un-bannered
case needed anything.

`rankPenalty` now multiplies score and graph mass by 0.5 for such files, taken
as the STRONGER of it and the generated penalty rather than multiplied — one
property two signals see must not be charged twice. Detection is structural, not
by extension, and four conditions deep. Two of them were forced by measurement:
requiring every symbol to be type-level takes the corpus flag rate from 1-18%
(which swept in Kotlin sealed classes, Rust mod.rs re-exports and django's
locale tables) down to 0-4%; requiring that nothing depends on the file
separates an ambient shim from a working types module, and without it the rule
demoted displacement-ts's pipeline `types.ts` and broke the CG-31 gate.

A query that NAMES a declared type is exempt, so a question about a type still
reaches its declaration at full weight. Precise tokens only, so "…the file
body…" cannot exempt a `Body` interface it never meant to name; this needs its
own set because `namedSeedIds` is callable-only and a type never becomes one.

Regression evidence in docs/benchmarks/explore-declaration-only-cg28.md:
6-repo envelope sweep byte-identical against a clean baseline build, zero
ambient files reach the candidate set on VS Code across five queries, corpus
flag rate 0-0.74%, both allocation fixtures PASS, full suite 2,978 green.

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

src/mcp/explore-diagnostics.ts

@@ -66,6 +66,12 @@ export interface ExploreCandidateMeta {
   spine: boolean;
   lowValue: boolean;
   generated: boolean;
+  /**
+   * Nothing but type declarations in this file, and nothing in the index
+   * depends on it (CG-28) — it cannot answer a flow question, so it ranks on
+   * discounted signals unless the query named one of the types it declares.
+   */
+  ambientDeclaration: boolean;
   /**
    * Multiplier `rankPenalty` applied to BOTH `score` and `graphScore` (1 = no
    * penalty). Generated and test/i18n files rank on discounted signals, so the
@@ -579,6 +585,7 @@ export class ExploreDiagnostics {
           spine: r.spine,
           lowValue: r.lowValue,
           generated: r.generated,
+          ambientDeclaration: r.ambientDeclaration,
           penalty: round6(r.penalty),
           kinds: r.kinds,
           allowance: r.allowance,
@@ -796,5 +803,6 @@ function flagString(f: ExploreDiagnosticFile): string {
   if (f.spine) flags.push('spine');
   if (f.lowValue) flags.push('low-value');
   if (f.generated) flags.push('generated');
+  if (f.ambientDeclaration) flags.push('ambient-decl');
   return flags.join(' ') || '-';
 }