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/index.ts

@@ -1550,6 +1550,19 @@ export class CodeGraph {
     return this.queries.generatedPredicateFor(filePaths);
   }
 
+  /**
+   * A `(path) => boolean` ambient-declaration test over a BOUNDED candidate
+   * list: true for a file that declares nothing but types, originates no call
+   * edge, and that nothing in the index depends on — an ambient `.d.ts` of
+   * global shims, vendored typings, module augmentation (CG-28). Structural
+   * rather than extension-based, and deliberately narrow: see
+   * `QueryBuilder.getAmbientDeclarationPathsAmong` for why each condition is
+   * there, in particular why a `types.ts` the codebase imports is NOT flagged.
+   */
+  ambientDeclarationFilePredicate(filePaths: Iterable<string>): (filePath: string) => boolean {
+    return this.queries.ambientDeclarationPredicateFor(filePaths);
+  }
+
   /** How many indexed files are flagged tool-generated. Reported by `status`. */
   getGeneratedFileCount(): number {
     return this.queries.countGeneratedFiles();