docs(tmp): impression-tracker implementation reference (rework of #3359)

.changeset/idmatch-impression-tracker-impl-reference.md

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+---
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+Add `docs/trusted-match/impression-tracker-implementation.mdx` — non-normative implementation reference for the impression tracker that sits behind the cap-fire boundary contract. Covers cross-identity dedup via `impression_id`, the `fcap_keys` label model, the log-based reference data model from `adcp-go/targeting/`, SDK primitives (`decodeTmpx` + `writeExposure`), production topology, and two end-to-end conformance scenarios (multi-identity dedup and cross-seller advertiser cap). Cross-links from `identity-match-implementation.mdx` so readers can find it.
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+This re-introduces, as non-normative impl reference, the impression-tracker mechanics that were originally proposed as normative architecture in `bokelley/idmatch-design` but were superseded on `main` by the narrower cap-fire boundary contract (#4070). The boundary contract stays normative; this page documents one valid way to implement the impression tracker behind it.

docs/trusted-match/identity-match-implementation.mdx

@@ -109,6 +109,7 @@ Today the cap-state store is keyed at `(user_identity, seller_agent_url, package
 ## See also
 
 - [TMP Specification](/docs/trusted-match/specification) — wire spec, TMPX format, conformance invariants
+- [Impression Tracker Implementation Reference](/docs/trusted-match/impression-tracker-implementation) — non-normative reference for the impression-tracker side of the boundary (multi-identity dedup via `impression_id`, fcap_keys label model, log-based reference data model, SDK primitives)
 - [Buyer Guide](/docs/trusted-match/buyer-guide) — buyer agent integration, Context Match + Identity Match flows
 - [Migration from AXE](/docs/trusted-match/migration-from-axe) — for buyers transitioning from AXE-shaped pipelines, including the OpenRTB User.eids cross-walk
 - [Privacy architecture](/docs/trusted-match/privacy-architecture) — what each party learns