document and test matched_by on agency-filter diagnostics

[makegov-mark]

What

Tango API 5.9.0 adds matched_by to each entry of meta.resolved_filters.<filter name>[] that resolved, saying how the token matched:

• key: an organization UUID.
• code: a 3-digit CGAC or 4-digit FPDS code.
• name: the organization's name, including a department's everyday name, a spelling variant or a rename.
• alias: an abbreviation or the organization's own alias.
• fuzzy: a looser text match, worth checking against the resolved name.

An entry that did not resolve has no matched_by.
The change is additive.

Why no behaviour change

PaginatedResponse.meta is a plain Record<string, unknown> passed through as the API sends it, so matched_by already reaches callers.
This PR only documents the field and pins it with a test.

Changes

• src/types.ts: the meta TSDoc describes matched_by and its five values.
• docs/API_REFERENCE.md: the five values and a short example that flags fuzzy matches.
• tests/unit/client.meta-diagnostics.test.ts: one test that a response carrying matched_by round-trips through res.meta, and that unresolvedAgencyTokens and resolvedAgencies are unchanged by it.
• CHANGELOG.md: entry under [Unreleased].

Testing

• npm run lint
• npm run typecheck
• npm run build
• npx vitest run (437 passed)

Not in this PR

• No version bump; that belongs to the release PR.
• No new parsed view. resolvedAgencies returns only the matched organizations, so matched_by is read from res.meta directly. A view that exposes it (for example one listing loosely matched tokens per filter) would be new public surface and is left as a follow-up decision.

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