[M0-EXIT-01] M0 exit gate: verify all M0 steps, record evidence, mark milestone complete

[offdev]

M0-EXIT-01 — M0 exit gate

All 21 prior M0 steps re-verified against their Scope/Verify clauses and the PRD; the three exit criteria are confirmed and recorded.

Verification performed (on commit 829026f)

Local (this machine):

• Fresh canonical build tree (g++ 16.2.1): configure + build zero warnings, ctest 32/32
• build-shared (NFR-8.9), build-asan (ASan+UBSan fatal), build-tsan (TSan halt_on_error=1), fresh Clang 22.1.8 tree: all ctest 32/32
• laige-fuzz json_parse --runs=1000 clean; laige-detcheck --scenario=synthetic OK; laige-api-scanner --check laige-api.json up to date (376 symbols); tools/laige-include-lint OK (count: 1 (budget: 10, PRD §11))
• Vendored-tree lock re-proven live: tampered deps/googletest file fails the configure with expected-vs-actual hashes; restored tree passes

CI (gate evidence):

• Merge-lane run https://github.com/offdev/laige-cpp/actions/runs/34749756361 on 829026f: all 10 jobs success, each under 1.2 min (10-min PRD §8.1 budget)
• ctest 32/32 passed in every P0 OS job (linux-gcc, linux-clang, linux-asan+UBSan, linux-tsan, windows-msvc, macos-arm64, macos-intel)

Changes in this PR

1. docs/benchmarks/baselines/m0-synthetic.md — first baseline file: full AGENTS §12 metadata (hardware, OS, compiler, build type, flags, workload, warm-up, n, stats, before/after), both verbatim runs, exact commands + commit. The synthetic stand-in measures no physical quantity, so no budgets.json measured field is updated (all 15 entries keep measured: 0 until their subsystem lands in M1+).
2. docs/benchmarks/baselines/README.md — stale "no baselines yet" line replaced with a pointer to the first baseline.
3. README.md — status line: M0 complete.
4. roadmap/M0-foundations.md — M0-EXIT-01 checked, with Decision/Verify evidence (CI run URL, per-OS ctest summaries, baseline link).
5. roadmap/README.md — Progress Board M0 22/22 complete, Total 22, change-log row.

Docs-only change (two commits: baseline + status updates, then the roadmap records). No code, no flag, no budget movement.