fix(backlog): #320 declared no status and red main

[wshallwshall]

main is red at ea055251 and this is the one-line fix. Verified by running the guard on a clean checkout of main, not just on a branch.

BACKLOG.md:7299: item #320 declares no status. Add exactly one leading banner:
'> ✅ **SHIPPED …**', '> ⛔ **DECLINED …**', '> 🪦 **RETIRED …**',
'> 🔢 **Re-scored …**', or '> 🚧 **Status …**'.

My fault, and a dull one: I opened #320 with 📋 **Filed 2026-08-01, not started.** — I invented an emoji instead of using the vocabulary the invariant defines. Corrected to 🚧 **Status: OPEN INVESTIGATION …**, which is what the item actually is.

tests/test_backlog_status_check.py — 15 passed, 0.4s.

Why this is worse than a typo

It fails in 0.6 seconds and runs inside every required test leg, so it reds every PR that compiles the suite. It was already blocking auto-merge on #119 and would have blocked #118. A trivially-detectable defect in a docs file took main down for every session in the repo.

How it merged green — a real coverage hole

#117 was docs-only, and ci.yml's changes job short-circuits exactly that case: when changes.outputs.code == 'false' the install/lint/type/test steps are skipped and the required test (…) contexts go green in seconds.

So the guard that polices BACKLOG.md does not run on a PR that only changes BACKLOG.md. That's the gap: the one class of change this invariant exists to catch is precisely the class that skips it. It isn't theoretical — it just happened, and it was found by another session's PR tripping over it rather than by mine.

I'm not fixing that here; this PR restores main and nothing more. Worth a follow-up decision: either add docs/BACKLOG.md to the paths that count as code for the short-circuit, or run the ledger/backlog guards in a cheap always-on leg that the docs-only path cannot skip. The second is closer to how backlog-hygiene.yml already behaves.

Also worth noting

The banner now carries the two facts a reader of #320 needs up front: measurement tooling landed in #118, and the decisive experiment is blocked because the self-hosted WS2025 runner is unregistered (actions/runners → total_count: 0, selfhosted-win2025-sql.yml never run).