fix(buzz-acp): thread cache-read tokens into NIP-AM kind:44200 events

[wpfleger96]

Problem

buzz-agent measures and sends accumulatedCachedInputTokens on the wire (usage.rs:93). buzz-acp deserializes it correctly — but then drops it: TurnUsage had no cache field, and build_turn_metric_counts hardcoded cache_read_tokens: None and cache_write_tokens: None into both turn and cumulative TokenCounts. Every kind:44200 event published permanently lacked data the harness measured. The archive is append-only — this is unrecoverable data loss per turn, every turn, until fixed.

NIP-AM already specifies the fields (cacheReadTokens / cacheWriteTokens inside turn and cumulative). This is a pure threading fix.

Changes

crates/buzz-acp/src/usage.rs

• SessionState gains last_cached_input: u64 to track the committed cache-read baseline.
• TurnUsage gains turn_cache_read_tokens: Option<u64> (field-local; None when no baseline or counter decreased) and cumulative_cache_read_tokens: u64 (always present; zero when no cache hits reported).
• record() computes the cache-read delta with field-local taint semantics: a decrease in the cumulative counter nulls only turn_cache_read_tokens — it does not flip delta_reliable or invalidate turn_input_tokens/turn_output_tokens. Identical to the accumulatedTotalTokens pattern already present.
• take() and the setup-notification branch both advance last_cached_input in the committed baseline.

crates/buzz-acp/src/pool.rs

• build_turn_metric_counts wires turn_cache_read_tokens into turn.cache_read_tokens (when delta_reliable) and Some(cumulative_cache_read_tokens) into cumulative.cache_read_tokens.
• cache_write_tokens remains None on both counts with an explanatory comment: buzz-agent does not emit a write-side count on the wire today.
• Six existing TurnUsage struct literals in tests updated with the two new fields.

Tests

usage.rs — new cache-read section (5 tests):

• cache_read_first_turn_produces_none_turn_delta_and_passes_cumulative_through — no baseline → delta None, cumulative passes through
• cache_read_second_turn_delta_computed_correctly — delta = current − previous
• cache_read_decrease_nulls_turn_cache_but_leaves_delta_reliable — field-local taint: decrease nulls cache delta only, input/output stay reliable
• cache_read_zero_payload_after_baseline_produces_zero_delta — zero on both sides → Some(0), not None
• cache_read_threads_through_setup_notification_baseline — setup notification baseline correctly seeds the cache counter

pool.rs — new acceptance test (1 test):

• test_build_turn_metric_counts_cache_read_tokens_thread_through — wire-parses a buzz-agent payload with nonzero accumulatedCachedInputTokens, runs two turns through the tracker and build_turn_metric_counts, and asserts nonzero cacheReadTokens in cumulative + correct per-turn delta in turn; also asserts cache_write_tokens is None throughout

Quality gates at tip c6405eb43f532572e3b7775e0dee826dc9cb3f82

Gate	Result
cargo test -p buzz-acp	655/655, 0 failed
cargo clippy -p buzz-acp --all-targets -- -D warnings	clean
cargo fmt --check	clean

Note: the pre-push hook mobile-test gate fails on origin/main before this branch (Flutter test in channels_page_test.dart / compose_bar_test.dart — verified independently). My changes touch only crates/buzz-acp/src/; the mobile failure is unrelated and pre-existing.