[manager/meta] support V6D and subscriber reporting for multi-engine instances

[lucky-zzz]

Summary: support ranked reporter identities for multi-engine instances; project shared V6D cache locations onto active engine reporters while preserving the physical Vineyard peer; cover normal, non-shared V6D, and shared V6D reporting scenarios. Test: bazel test //kv_cache_manager/... (111 passed, 1 skipped).

[qoderai]

Review Summary

Overall assessment: Solid, well-scoped change. The rank-suffix reporter identity, the shared-L2 → active-L1P5 projection, and the physical-vs-logical split for Vineyard peers are all implemented cleanly and covered by a comprehensive test. I don't see correctness bugs; the observations below are minor.

Lifetime / pointer safety

• HostCacheLocationInfo::logical_hosts is a raw const std::vector<std::string> *, but it points into EventVisibilitySnapshot::logical_hosts_by_reporter, which lives inside the event_snapshots shared_ptr captured by the checker lambda. Since VisitHostSpecsForOneKey uses location_info only within the same callback invocation, the pointer is valid for its entire use. ✓
• reporter_host is a std::string_view into the snapshot's location-id buffer; the snapshot is held alive by the same shared_ptr capture. ✓

ParseReporterIdentity

• The rejection rules (empty rank, leading zero, duplicate @, non-digit, trailing chars) are all covered by the test cases @, @-1, @rank, @00, @0@1.
• Using uint64_t for rank matches the rest of the codebase and avoids surprises for very large ranks.
• One tiny nit: in the caller at cache_manager.cc:2705, ReporterIdentityView reporter_identity; is default-initialized and then passed by reference; since its value isn't used downstream of ReportEvent, consider dropping the local and passing an anonymous temporary, or at least marking it (void) to make the "validation-only" intent obvious to readers.

Projection logic (cache_manager.cc:4667-4702)

• Building ranked_hosts_by_base from L1P5 and then filling logical_hosts_by_reporter per storage type is correct. The L2-only projection condition (parsed && storage_type == L2 && !identity.engine_rank.has_value()) matches the design: only a shared, unranked L2 reporter fans out to every active ranked engine.
• The sort + unique on ranked_hosts is defensive; ranked_hosts_by_base is built by iterating each reporter once, so duplicates aren't expected in normal operation. Fine as-is.
• Style nit: in for (auto &[base, ranked_hosts] : ranked_hosts_by_base) { (void)base; ... }, the binding could be const auto &[base, ranked_hosts] (and drop the (void)base) since neither is mutated, or just use auto &[_, ranked_hosts] if the toolchain supports C++26 structured bindings with _. Same for the (void)state; silencers.

Visitor signature change (meta_searcher.cc)

• Adding a fourth physical_reporter argument and keying vineyard_host_specs by physical_reporter is the correct move: a shared L2 reporter projected onto N logical ranks must still yield exactly one Vineyard P2P peer, not N. The new comment at meta_searcher.cc:626-628 explains this well.
• For non-event-report locations, the caller passes std::string_view(host) twice (logical and physical), which preserves prior behavior. ✓
• All four existing callers (BuildHostsForOneKey, BuildCandidatePresenceForOneKey, BuildHostSpecNamesForOneKey, PrefixMatchWithMambaByHostWithoutP2P) are updated; VisitHostSpecsForOneKey is a file-local template so no out-of-TU breakage is possible. ✓

Tests

• Three cases (ordinary / multi-engine independent / multi-engine shared) plus invalid-reporter coverage and a rank-lifecycle sequence. The GetCacheLocationsByBackend assertion in Case 3 is the right check that the data-access path does not get rewritten by the query-only projection — good regression guard.
• One optional gap: no test exercises L1P5 shared (unranked) with ranked L2. That configuration isn't part of the design (the asymmetry is intentional: L1P5 = subscriber identity, L2 = shared Vineyard), so this is more of a "nice to have" negative test to lock in that no projection happens in the reverse direction.

Non-blocking observations

• host_ip_port is reused as both a location-id component and a reporter identity. The @ character is accepted by SnapshotUriUtils::IsValidLocationIdComponent (otherwise the parse wouldn't see ranked input), worth a quick mental check that no downstream URI/snapshot builder chokes on it. The test URIs use event_report://host:port/mem without @, so the location URIs themselves stay clean.
• The PR description says "111 passed, 1 skipped"; consider running the integration test suite too (//integration_test/...) if not already done, since meta_searcher.cc is exercised by cross-module scenarios.

No blocking issues; feel free to merge once the (optional) nits are considered.

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