docs: add OPSTACK-HANDOFF.md; install the 39 Unreal skills

Handoff: a pasteable resume prompt at the plugin root -- read order, exact state
(Phase 0.5 green, Phase 1 done and measured, step 3 written but not compiled),
the immediate next action, hard rules, open items, and the method lesson from
this session.

Skills: Jahni's UE library was at .claude/skills/core/<name>/SKILL.md, two levels
deep, where Claude Code discovers skills one level deep -- so none of the 39 were
loading. Flattened; 124 reference files intact, all frontmatter valid, folder
names already matched their name: field. core/category.md left as documentation.
Confirmed loading.

They are untracked and cannot be tracked without un-ignoring .claude/ itself
(git cannot re-include a file whose parent directory is excluded). Same shape as
AUDIT P1; flagged, not actioned.

Note for the next session: module-and-build-system documents PCHUsage, shared
PCHs and IWYU -- the exact mechanism that blocked C10's settling experiment. That
skill was in the repo, undiscovered, while it was worked out the slow way.

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

OPSTACK-HANDOFF.md

@@ -0,0 +1,103 @@
+# Handoff — VoxelForge operator stack, 2026-07-27
+
+> Paste the block below into a fresh session. Everything it refers to is on disk and in git.
+
+---
+
+You're picking up an agreed refactor of the VoxelForge UE5 voxel plugin, on branch `experimental`
+(already checked out — do not create another). I'm Jahni. Design and progress are written down so you
+don't re-derive them.
+
+## Read first, in this order
+
+1. **`CLAUDE.md`** — project rules. **Rule #1 is absolute: never build, compile, or run the editor.**
+   I build everything myself. When code is done, stop, say "ready to build", list the likely
+   compile-error spots, and wait.
+2. **`OPSTACK-PROGRESS.md` — THE LAST ENTRY FIRST.** Append-only log of what is built vs merely
+   written. This is the resume point.
+3. **`OPSTACK-PLAN.md`** — the plan. §2.5 op taxonomy, §2.6 acceptance bar, §4 phases, §5 invariants.
+4. **`OPSTACK-DECOMPOSITION.md`** — all 8 archetypes broken into ops, every param traced. §10.4 is
+   the port order, §11 the open questions.
+5. **`AUDIT-2026-07.md` §C10** — read before touching the Maze equivalence test. Six builds were
+   spent there; it is parked deliberately.
+6. **`CODEMAP.md`** — navigation. Trust symbol names over line numbers.
+
+## Where things stand
+
+**Phase 0.5 — DONE, green.** Five automation tests, all passing (Session Frontend → Automation,
+filter `VoxelForge`): `DensityPurity`, `ClassifyTileSoundness`, `DiffLayerContention`,
+`LiveEditInvalidation`, `OpStack.BoxVerdictFold`, `OpStack.MazeEquivalence`. They found and fixed a
+real bug (AUDIT C2 — five caches missing `LayoutVersion`).
+
+**Phase 1 — DONE and verified.** `Maze` decomposes into **seven** ops with no contortion:
+`ConstantRock → LatticeCorridor → SdfRoughness → SdfCarve → Spine → Seal → Passage`. Measured:
+SDF reproduced **bit for bit**, **zero isosurface crossings** out of 20 000, window-invariant across
+worker threads, every box verdict survives brute force, and **23 of 60 tiles proved uniform** where
+`ClassifyTile` proves zero for any cave archetype. That last number is the perf case for the whole
+refactor, measured rather than argued.
+
+**Phase 1 step 3 — WRITTEN, NOT COMPILED.** The stack is wired into `GetDensityAt` behind
+`UVoxelStrateDefinition::bUseOperatorStack`, with the ported-archetype list living only in
+`UVoxelStrateManager::UsesOperatorStackForChunk()`. **This is the immediate next thing to build.**
+
+### The immediate next action
+
+1. I build. Likely error spots: the `else switch` form in `GetDensityAt`, `FVoxelOpStack` as a
+   `thread_local` (it is move-only, reset by move-assigning a temporary), the new include.
+2. Then the **visual A/B**, which is the last thing Phase 1 needs: tick `bUseOperatorStack` on a Maze
+   strate, regenerate, compare against the same seed with it off. **Pass = recognisably the same
+   maze** (same corridor scale, connectivity, feel). That is `OPSTACK-PLAN §2.6`'s bar — judged on a
+   screenshot, not a diff.
+3. Then **Phase 2**, starting with the cheapest win: `FlatPlain` + `CrystalChamber` collapse into one
+   op (`OPSTACK-DECOMPOSITION §3`). Ask me §3.1 first — whether the slab floor noise's Z term is
+   intentional; if it can go, both archetypes become XY-pure and start skipping tiles.
+
+## Hard rules that prevent real bugs
+
+- **Density sign:** negative = solid, positive = air *at the mesher*. Inside the op stack the
+  convention is INTERNAL (**positive = solid**), negated once by the caller. The SDF channel uses
+  standard SDF convention, so `min()` means opposite things on the two channels. This is the #1
+  source of confusion in the plugin — `VoxelDensityOp.h` explains it.
+- **Never run both density paths in one world**, and never compare them for equality (§C10).
+- Every cache key includes `LayoutVersion`; `ProcessQueue` stays `EQueueMode::Mpsc`; `Epoch` carries
+  through every async path; don't "optimize" the `ARCHITECTURE §8.10` perf invariants.
+- Never edit `Binaries/`, `Intermediate/`, `*.generated.h`. Comments are French + English — match the
+  surrounding file.
+- Commit per coherent unit with a real message. Never push. `main` is the known-good fallback.
+- Update `CODEMAP §3` for new/renamed symbols, `ARCHITECTURE §8` for design changes, tick phases in
+  `OPSTACK-PLAN.md`, and append to `OPSTACK-PROGRESS.md`.
+
+## ⚠️ The lesson from this session — it cost six builds
+
+Chasing a 1-ULP difference in the Maze port, I proposed **five** hypotheses and every one was refuted
+by measurement: the `FVector` round-trip, a transcription slip, `/fp:fast` across translation units,
+inlining context, and compile-time-constant vs runtime parameters. Each was plausible. Each cost a
+build. What actually produced information was instrumenting — a bisect, a three-way comparison, a
+variable-isolation harness.
+
+Worse, twice I reasoned confidently from a premise I hadn't checked (a build flag that had landed in
+the wrong module; a comparison that was circular by construction and could not fail).
+
+**So: on any numeric discrepancy, instrument before hypothesising, and verify the premise before
+reasoning from it.** And know when to stop — Jahni called it, correctly, once the port was proven
+correct on every axis that affects the game and only the last rounding was unexplained.
+
+## New this session: 39 Unreal skills are installed
+
+`.claude/skills/<name>/SKILL.md` — they were nested one level too deep and invisible; now flattened
+and loading. Directly relevant: `automation-and-testing`, `coding-standards`,
+`module-and-build-system` (covers `PCHUsage` / shared PCH / IWYU — the exact thing that blocked
+§C10's settling experiment), `game-thread-performance`, `memory-and-gc`, `navigating-engine-source`.
+**Use them** — the one that would have explained the shared-PCH failure was sitting in the repo,
+undiscovered, while I worked it out the slow way.
+
+## Known open items, deliberately not done
+
+- **AUDIT C1** — unbounded `SeedF`, one line at six sites. Dormant at small seeds; applying it
+  re-rolls the world's noise and forces a re-tune, so it wants its own build. **Still a real bug.**
+- **IWYU debt** — seven headers use engine types they never include; the plugin only compiles because
+  of the shared PCH. Blocks `FPSemantics` experiments. Worth clearing on its own terms.
+- **AUDIT C10** — the parked ULP residue. Read it before reopening; five hypotheses are already
+  refuted in a table there.
+- `OPSTACK-DECOMPOSITION §11` — five design questions for me, most importantly whether the two-channel
+  `Eval` (already implemented) stays.

OPSTACK-PROGRESS.md

@@ -811,3 +811,28 @@ connectivity, same feel. That is §2.6's bar, and it is the last thing Phase 1 n
 `OPSTACK-DECOMPOSITION §10.4` starts with FlatPlain + CrystalChamber collapsing into one op.
 
 ---
+
+## 2026-07-27 — session handoff written. 39 UE skills installed.
+
+**`OPSTACK-HANDOFF.md`** added at the plugin root: a pasteable resume prompt for a fresh session.
+Read order, exact current state, the immediate next action, the hard rules, the open items, and the
+method lesson from this session.
+
+**39 Unreal skills installed.** Jahni dropped a UE skills library into `.claude/skills/`, but nested
+as `.claude/skills/core/<name>/SKILL.md` — two levels deep, where Claude Code discovers skills one
+level deep at `.claude/skills/<name>/SKILL.md`. None were being loaded. Flattened (39 SKILL.md, 124
+reference files, all frontmatter valid, folder names already matched `name:`); `core/category.md`
+left in place as documentation. Confirmed loading.
+
+⚠️ **They are untracked**: `.gitignore` starts with `*`, and git cannot re-include a file whose parent
+directory is excluded, so nothing under `.claude/` can be tracked without un-ignoring the directory
+itself. Same shape as AUDIT P1. Flagged to Jahni, not actioned — vendoring a reference library into
+the plugin repo is his call.
+
+**Immediately relevant to open work:** `module-and-build-system` documents `PCHUsage` / shared PCH /
+IWYU — i.e. the exact mechanism that blocked §C10's settling experiment. That skill was in the repo,
+undiscovered, while it was worked out the slow way. Worth reading before clearing the IWYU debt.
+
+**Next single action unchanged:** build Phase 1 step 3, then the visual A/B on a Maze strate.
+
+---