PR-A4 of the Realignment Phase A lockdown
(REALIGNMENT/03-phase-A-lockdown.md). Eliminates P0-3 (Rust proxy
ignores customer cache_control markers) and P0-5 (numeric precision
lost via serde_json::Value round-trip) at the library level; Phase B
PR-B2 wires the helper into the live-zone block dispatcher.
Cargo.toml — add `arbitrary_precision` and `raw_value` to
`serde_json` workspace features. `arbitrary_precision` keeps `1.0`
from collapsing to `1` and preserves >2^53 integers; `raw_value`
exposes `&RawValue` so PR-B2 can forward unmodified `messages[*]`
entries as exact byte copies.
crates/headroom-core/src/cache_control.rs (new) — `compute_frozen_count`
walks `messages[i].content[*].cache_control` via serde_json
accessors only (no regex) and returns the smallest N such that
`messages[i]` is frozen for every i < N. Markers in `system` or
`tools[*]` log at debug! but never bump the floor (those fields are
unconditionally cache-hot per invariant I2). TTL ordering violations
(5m before 1h, guide §2.19) emit `tracing::warn!` but the function
computes the correct count regardless — the customer's request, not
ours to reject.
crates/headroom-core/src/lib.rs — re-export `compute_frozen_count` at
crate root so the proxy crate has a stable import path.
crates/headroom-proxy/src/compression/anthropic.rs — add
`resolve_frozen_count` thin wrapper that consults the
`cache_control_auto_frozen` config flag. When `disabled`, returns 0
regardless of body content (operator opt-out for benchmarking).
crates/headroom-proxy/src/config.rs — add `CacheControlAutoFrozen`
enum and the matching CLI flag `--cache-control-auto-frozen` /
env var `HEADROOM_PROXY_CACHE_CONTROL_AUTO_FROZEN`. Default is
`enabled`. Documented in the doc comments.
Tests
- crates/headroom-core/src/cache_control.rs (inline): 11 unit tests
covering marker detection, system/tools negative cases, ordering
state machine, defensive (missing fields, non-array messages,
non-object content blocks).
- crates/headroom-core/tests/cache_control.rs: 11 unit + 3 property
tests (monotonic non-decrease as markers are added; system/tools
markers don't change count; empty messages → 0).
- crates/headroom-proxy/tests/integration_cache_control.rs: 8 tests
exercising the proxy wrapper (configurability gate; tracing
capture for the 5m-before-1h warn path).
Acceptance gates: `cargo build --workspace`, `cargo test --workspace`
(33 new tests green), `cargo clippy --workspace -- -D warnings`,
`cargo fmt --all --check` all clean. No new `regex::` imports;
`git grep -n 'regex::' crates/{headroom-core/src/cache_control.rs,
headroom-core/tests/cache_control.rs, headroom-proxy/tests/
integration_cache_control.rs}` empty.
Honors the realignment build constraints: configurable (CLI + env),
no hardcodes (TTL strings live as const), no regex (serde_json
accessor walk), no fallbacks (one impl), structured logging
(debug!/warn! with field/index/ttl/rule context), tests
comprehensive (unit + property + integration + tracing capture).
