docs(tracking): RM-400 Campaign 4 final-gate evidence audit [DEV-78]

[P4suta]

Adds docs/tracking/campaign-4-evidence.md, an audit-only snapshot of the RM-400 (DEV-78) Campaign 4 final completion gate. Linear: DEV-78. It closes/advances nothing — per the roadmap decision rule, a checkbox flips only on its own durable evidence, never by date/version, and release execution stays a manual maintainer decision.

What it records

Each of the ten RM-400 required-evidence checkboxes is audited against the CI in .github/workflows/ci.yml and the roadmap, marked present / partial / not started with the concrete enforcing gate (CI job, xtask gate, ADR) named.

• Tally: 3 present / 5 partial / 2 not started — the gate is not met.
• DEV-124 (blocker) has landed (fix(xz): deterministic async XZ decoder wakeups, no worker deadlock [DEV-124] #70) → checkbox 9's flaky-hang blocker is cleared; the covered CI axes now need only the acceptance's sustained ≥20 hang-free reps recorded. RM-303 7z depth landed (feat(7z): coder-graph depth — multi-folder, BCJ/Delta, Deflate/BZip2/Zstd, AES-256 [RM-303] #71) → strengthens checkbox 5 interop (PPMd/BCJ2 still deferred).
• Still-open: C ABI freeze = RM-310 not started, three-producer/two-consumer completeness, 10 GiB soak, a CI perf-regression gate, WASI/32-bit axes, and ≥2 RCs.

No tag/version/release language.

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