chore: track the design docs in git (.gitignore !*.md)

AUDIT P1: every markdown design doc except CODEMAP.md was untracked, so
ARCHITECTURE / AUDIT / OPSTACK-PLAN / fable-idea / REVIEW_FINDINGS lived
only on disk. Replaces the single !CODEMAP.md exception with !*.md.

Also makes OPSTACK-PROGRESS.md commits actually record something, which
the unattended crash-safety discipline depends on.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

OPSTACK-PROGRESS.md

@@ -0,0 +1,36 @@
+# OPSTACK — progress log
+
+> **APPEND ONLY. Never rewrite or reorder entries.** Write the entry for a piece of work *before*
+> starting it, so an abrupt session end still leaves an accurate marker.
+>
+> **Entry format:** date · what · believed-true · **UNVERIFIED** (everything not yet built by Jahni) ·
+> next single action.
+>
+> This file is how a fresh context resumes. Read the last entry first, then `OPSTACK-PLAN.md §9`.
+
+---
+
+## 2026-07-27 — branch created, design locked, nothing built
+
+**What:** Branch `experimental` created from `69fa73e tmp` and checked out. `main` untouched and is the
+known-good fallback world. Design finalised across `OPSTACK-PLAN.md` (incl. §2.5 op taxonomy and §2.6
+acceptance bar, both added after Jahni's objection that a naive "ops" reading would just be his old
+room-operations system). Kickoff prompt written to `OPSTACK-PROMPT.md`.
+
+**Believed true:** the plugin builds and runs correctly as of this commit — everything through F20
+phases 1+2 is built and working (confirmed by Jahni 2026-07-26; the "PENDING BUILD" markers still in
+`fable-idea.md` / `ARCHITECTURE.md` are stale and are queue item Q3). `bEnableDensityVolume` is already
+set to false. Lighting is deprecated for now. Live direction before this refactor was 3D density
+generation (overhangs, caves inside mountains).
+
+**UNVERIFIED:** nothing yet — no code has been written for this refactor.
+
+**Known open bugs, not yet fixed, documented in `AUDIT-2026-07.md §1`:** C1 large-seed noise collapse ·
+C2 three cache keys missing `LayoutVersion` · C3 `FMath::PerlinNoise2D` on the SDF hot path (dormant) ·
+C4 `GetPlayerPosition` zero-vector sentinel · C5 unbounded-join-on-shutdown · C6 diff-layer scaling.
+
+**Next single action:** Phase 0.5 — the three automation tests (density purity across worker threads,
+`ClassifyTile` vs brute force, `DiffLayer` under contention). Then the Phase 1 skeleton header only.
+Then STOP for a build. See `OPSTACK-PROMPT.md` → `WHAT TO DO`.
+
+---