feat(rust): scaffold workspace + parity harness (phase-0)

Bootstrap the Rust port of Headroom. Additive only — no existing Python
code modified. Ships the workshop, not the widgets.

Layout
  Cargo.toml (workspace) + rust-toolchain.toml
  crates/headroom-core    — transform library, stub only
  crates/headroom-proxy   — axum binary, /healthz only
  crates/headroom-py      — PyO3 cdylib, exposes headroom._core.hello()
  crates/headroom-parity  — Rust-vs-Python oracle harness + parity-run CLI

Tooling
  Makefile: test, test-parity, bench, build-proxy, build-wheel, fmt, lint
  .github/workflows/rust.yml: test, wheels (linux/mac), audit, parity-nightly
  deny.toml for cargo-deny

Parity corpus
  tests/parity/recorder.py + scripts/record_fixtures.py
  125 recorded fixtures across 5 leaf transforms (ccr, tokenizer,
  log_compressor, diff_compressor, cache_aligner)

Docs
  RUST_DEV.md — developer setup and workspace reference
  docs/spec/022-rust-migration.md — migration plan and stage breakdown

.gitignore: whitelist scripts/record_fixtures.py; ignore target/

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/record_fixtures.py

@@ -0,0 +1,59 @@
+#!/usr/bin/env python3
+"""Entry point for recording Rust-vs-Python parity fixtures.
+
+Usage:
+    python scripts/record_fixtures.py
+
+This driver:
+  1. Monkey-patches the Phase-1 transform classes via
+     `tests.parity.recorder.record_all()`.
+  2. Runs a small deterministic synthetic workload (no network, no real LLM
+     calls) that hits each transform at least 20 times with varied inputs.
+  3. Prints a summary and exits non-zero if any transform was blocked.
+"""
+
+from __future__ import annotations
+
+import logging
+import sys
+from pathlib import Path
+
+# Make `tests/parity/recorder.py` importable without installing it.
+_REPO_ROOT = Path(__file__).resolve().parent.parent
+sys.path.insert(0, str(_REPO_ROOT))
+
+from tests.parity.recorder import record_all, run_default_workload  # noqa: E402
+
+
+def main() -> int:
+    logging.basicConfig(level=logging.INFO, format="%(levelname)s %(name)s: %(message)s")
+    log = logging.getLogger("record_fixtures")
+
+    statuses = record_all()
+    log.info("patch status:")
+    blocked = []
+    for name, status in statuses.items():
+        log.info("  %-16s %s", name, status)
+        if status.startswith("blocked:"):
+            blocked.append((name, status))
+
+    counts = run_default_workload()
+    log.info("fixture counts:")
+    shortfall = []
+    for name, n in counts.items():
+        log.info("  %-16s %d", name, n)
+        if n < 20:
+            shortfall.append((name, n))
+
+    if blocked:
+        log.error("blocked transforms: %s", blocked)
+    if shortfall:
+        log.error("transforms with <20 fixtures: %s", shortfall)
+
+    # Exit non-zero if anything was wholly blocked; short recordings are
+    # a soft warning.
+    return 1 if blocked else 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())