[M0-TOOL-02] Determinism checker skeleton: laige-detcheck + scenario contract + CI job

[offdev]

What

M0-TOOL-02 (FR-11.5, AGENTS ARCH-010, TEST-004): the determinism
checker skeleton. laige-detcheck runs a named scenario in two build
configurations and compares per-tick state hash streams. The engine
state-hash API arrives with M1-DET-03, so the tool works against the
documented hash-line output contract instead:

• Scenario contract (normative text in the tool header, mirrored in
  docs/api/detcheck.md): one stdout line per tick <tick> <hash> —
  tick starts at 0, step 1, no padding; hash = exactly 16 lowercase
  hex digits; trailing newline optional, trailing \r tolerated
  (Windows CRLF); stderr ignored; exit 0. The hash algorithm is not
  part of the contract — detcheck compares lines byte-for-byte, so
  only run-to-run identity matters.
• Strict + bounded enforcement (CORE-008): a malformed line, tick
  gap, > 65536 ticks, or > 64 bytes/line is a loud exit-2 error — a
  broken scenario is never a silent mismatch.
• Report (stable, machine-greppable — LOG-001):
  detcheck scenario=<name> result=OK ticks=<n> or
  result=DIVERGED first_diff_tick=<t> + run-a/run-b lines (the
  diverging tick pair, or stream ends: <n> ticks when one run ends
  early).
• Exit codes: 0 = match · 1 = divergence (loud) · 2 = usage /
  unknown scenario / scenario failure / contract violation.

Modes

Command	Meaning
laige-detcheck --scenario=synthetic [--ticks=N] [--seed=X]	built-in 32-body fpx16_16 + Prng workload, two in-process runs — pure integer arithmetic, bit-exact across build/platform/ISA/compiler (ADR 0002)
laige-detcheck --scenario=synthetic-perturbed	the same, but run-b adds 1 unit to body 3's x at tick 7 — the perturbation fixture that proves the failure path
laige-detcheck --run-a=<binA> --run-b=<binB> [-- scenario-args...]	the real mode from M1-DET-04 on: two builds of one scenario compared; -- keeps scenario args unambiguous (strict surface, API-008)

Scenario execution: fork/exec + pipe capture (POSIX), CreateProcessW +
PeekNamedPipe (Windows) — no shell, bounded memory, the scenario's
stderr stays visible in the CI log. Build type stamped into the report
labels (AGENTS §12) via LAIGE_DETCHECK_BUILD_TYPE.

Determinism scope (ARCH-010): same build/platform/ISA/compiler —
the built-in workload is pure unsigned-integer arithmetic
(fpx16_16 ops + xorshift128+), which the language standard makes
bit-exact; no float anywhere. M0 hash scope: tick + seed + body
words; the definitive scope (including Prng state and the exact hash
function) is defined by M1-DET-03's world.state_hash — the tool's
line-comparison is unchanged.

Tests (ctest -R detcheck, 9 entries)

The tool is tested with a synthetic two-run scenario (the step's
Verify clause): the built-in self-check, the built-in perturbation
fixture, and the cross-binary mode against fixture scenario binaries —
one source (tests/detcheck/detcheck-fixture-scenario.cpp), five
compiled variants (clean / perturbed / bad output / early exit / short
stream). Each test is a generated cmake -P check script asserting
BOTH the exit code and the required output fragments (the tests/api
pattern — CTest inverts PASS_REGULAR_EXPRESSION under WILL_FAIL,
and a crash exits non-zero like a correct failure, so the content must
be checked).

CI

New detcheck job in both workflows (every PR and merge, no ci:*
condition — a tooling check like include-lint and api-manifest,
not an additional P0 OS build): configure → build →
laige-detcheck --scenario=synthetic; the real-scenario step (two
build configurations of M1-SAMPLE-01's hello) is skipped until that
sample lands — M1-DET-04 activates it and records the result per
ARCH-010. The ci.yml header job count is fixed at the same time
(it said eight jobs before M0-TOOL-01 even landed; now ten, matching
the actual job list).

Docs

docs/api/detcheck.md (contract, CLI, report/exit codes, determinism
scope, CI status), building.md (current-status entry; the reserved
tool-note is now live), docs/README.md, tools/README.md, root
README.md.

Verification

• Local, all green, zero warnings: ctest 31/31 on g++ 16.2.1 Debug
  static, g++ shared (LAIGE_BUILD_SHARED=ON), clang++ 22.1.8 Debug,
  clang++ ASan+UBSan (LAIGE_ASAN=ON), clang++ TSan (LAIGE_TSAN=ON,
  TSAN_OPTIONS=halt_on_error=1).
• ./build/bin/laige-detcheck --scenario=synthetic → exit 0,
  result=OK ticks=256.
• ./build/bin/laige-detcheck --scenario=synthetic-perturbed →
  exit 1, result=DIVERGED first_diff_tick=7 (the step's Verify
  clause).
• Windows execution path: compile-verified by the CI MSVC job (not
  testable locally on Linux).

Also in this change

• Roadmap: M0-TOOL-02 checked (Decision/Verify/Size), board M0
  17 → 19 done.
• Change log: M0-TOOL-02 row + retroactive M0-TOOL-01 row
  (937ac7c/PR [M0-TOOL-01] Public API manifest: laige-api-scanner + checked-in laige-api.json + CI drift check #10 merged without updating the board or the log).
• building.md: fixed a missing line break in the current-status
  section (pre-existing typo in the section this step extends).