docs: spec CODEX-TASK-001 (tile-skip stats) and refresh the handoff for the Codex tandem

Jahni built a world and said "I don't know if it dropped any meshing? but it
looks alright by the eye." That sentence is the honest state of this refactor:
everything proved so far was proved in an automation harness on 40 sampled
tiles, and in the running game tile-skipping is unobservable.

grep INC_DWORD_STAT over Source/ returns nothing -- the plugin has zero stat
counters -- and "skipped correctly" renders identically to "skipped nothing", so
no visual check can separate them. The tests got the "coverage is a number, not
a boolean" discipline this session; the game never did.

CODEX-TASK-001-tile-skip-stats.md specs a stat VoxelForge group with
TilesClassified / TilesSkippedAllSolid / TilesSkippedAllAir / TilesMeshed. Solid
and air are split deliberately: cave archetypes prove AllSolid, so that counter
is the one that says whether the op-stack work did anything real. Its deliverable
is the before/after that constitutes the PRODUCTION proof of T1.d, which does not
exist today.

The spec carries the invariants rather than just the task, which is the point of
a spec here: bTrivialEmpty decides whether a tile has COLLISION, the five-clause
gate is load-bearing, and GenerateTileResult runs on worker threads -- so a plain
static int32++ is a data race while INC_DWORD_STAT is not.

Handoff updated: a "How we work now" section (Codex Model Luna xHigh writes most
code, Claude orchestrates -- hand over the INVARIANT, review against the code and
not the description), first actions split into the Codex task and the pending
e002bd4 build, and the T1.d heading qualified as harness-measured rather than
game-proven. Also records the Insights interim answer and its trap: ~84% of tiles
were already rejected by the hand-written surface/bedrock paths, so surface skips
drown the cave ones unless you are underground in an opted-in strate.

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

OPSTACK-PROGRESS.md

@@ -3266,3 +3266,67 @@ existing assertion fails rather than a player falling through the floor.
 stay 0. If it is still 0 proved, the warning now says what to check *first*: `ExtraReach` inflates
 both remaining tests, so its value against `ShaftMaxRadius` is the thing to look at before touching
 either test — not a re-derivation from scratch.
+
+## 2026-07-29 — clean stopping point. Codex joins; the production-observability gap is named.
+
+### The finding that ends this session
+
+Jahni built a world, looked at it, and said: *"I don't know if it dropped any meshing? but it looks
+alright by the eye."*
+
+That sentence is the honest state of this entire refactor. **Everything proved so far was proved in
+an automation harness, on 40 sampled tiles.** In the running game, tile-skipping is **unobservable**:
+
+- `grep INC_DWORD_STAT Source/` → **nothing**. The plugin has zero stat counters.
+- *Skipped correctly* and *skipped nothing* render identically, so no visual check can separate them.
+- And `bUseOperatorStack` defaults **false** with no strate ticked, so unless Jahni ticked one, the
+  honest answer is "nothing new was dropped, by construction".
+
+This is the same failure this session spent six rounds learning to avoid — *coverage is a number, not
+a boolean* — applied to production instead of to a test. The tests got that discipline; the game
+never did.
+
+**Interim answer that needs no code:** the trace scopes `VoxelForge_ClassifyTile` and
+`VoxelForge_GenerateMesh` already exist at the site (`AVoxelWorld::GenerateTileResult`,
+VoxelWorld.cpp ~1501). In Unreal Insights, a skipped tile is a `ClassifyTile` with no `GenerateMesh`
+after it. ⚠️ But the comment there records that **~84 % of tiles were already being rejected** by the
+hand-written SurfaceWorld/bedrock paths long before the op stack existed, so surface skips drown the
+cave ones — the number only means something **underground in an opted-in `TunnelNetwork` strate**.
+
+### `CODEX-TASK-001-tile-skip-stats.md`
+
+Written and committed. A `stat VoxelForge` group with `TilesClassified / TilesSkippedAllSolid /
+TilesSkippedAllAir / TilesMeshed`. Solid and air are split deliberately: **cave archetypes prove
+`AllSolid`**, so that counter is the one that answers whether the op-stack work did anything real.
+
+Its deliverable is a before/after that is **the production proof of T1.d**, which does not exist
+today: no strate opted in ⇒ `TilesSkippedAllSolid` underground reads 0; tick `bUseOperatorStack` on
+one `TunnelNetwork` strate, fly the same route ⇒ non-zero.
+
+The spec carries the invariants rather than just the task, because that is the whole point of a spec
+here: `bTrivialEmpty` decides whether a tile has **collision**, the five-clause gate is load-bearing,
+and `GenerateTileResult` runs on **worker threads** — so a plain `static int32++` is a data race and
+`INC_DWORD_STAT` is not.
+
+### Working arrangement, from now on
+
+**Codex (Model Luna, xHigh) handles most of the coding; I orchestrate** — read the code, decide what
+to do, write precise specs, review what comes back **against the code rather than its description**,
+and own the docs and the measurements. Recorded in the handoff (`## How we work now`) and in memory.
+
+### Git
+
+`experimental` is now **pushed and tracked** (`origin/experimental`, Jahni's Gitea). `main` stays
+pinned at the known-good commit. The old flat "never push" rule was corrected in both places that
+stated it — with the caveat that matters: **a pushed commit is not a "verified green" marker**, since
+this branch carries unbuilt work by design. `OPSTACK-PROGRESS.md` remains the only record of what was
+actually built.
+
+### Still open, in priority order
+
+1. **`CODEX-TASK-001`** — the production observability gap. Priority, because every claim this
+   refactor makes is currently harness-only.
+2. **Build `e002bd4`** (VerticalShafts connector capsules) and read
+   `Box verdicts over 60 VerticalShafts tiles`. 0 was the number for the project's whole life.
+3. **PERF** — the op path is measurably slower; suspects listed in the handoff. Measure first.
+4. The warp squeeze stays **parked** with its ceiling measured and a negative result recorded.