diag+docs: confirm grey-BG root cause = unfilled slot-table (label_125bd via stubbed coroutine op 0x140)

Traced end-to-end (AGE_DIAG_SETTEX): set-texture slot=G[0x62452] <- query-gfx-object?
(-1 for unregistered CG handles) -> fallback lookup-array-2d(rec[s3]=G[0x3239])=0
because the slot table is never filled: label_125bd (SC0000 0x50f, slots 4..13) is
gated behind the scene-coroutine framework (0x140 coroutine-yield, stubbed). Adds
env-gated VM set-texture/query slot diagnostics + GfxState.IsRegistered.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/engine-re.md

@@ -637,11 +637,20 @@ Diagnosed with the new `--gfx-log` compositor/op trace (docs/tools-reference.md)
    the effect spritesheet (`AE001H`, an 800×400 4×2 grid of blob frames) **all set-texture into slot 0**.
    Objects live-reference their slot, so loading the effect **evicts** the BG → grey; and the effect is
    drawn full-screen from slot 0 (its object `src=(0,0 800x600)`) → the whole sheet (blob grid) covers the
-   screen. ⇒ The layering failure is a **slot-assignment** problem: the slot-selecting globals aren't
-   populated (unseeded boot/gfx state, or a stubbed op the display subroutine relies on), collapsing the
-   scene into one slot. **Open (next):** trace an effect-display subroutine to find which global holds the
-   slot and why it's 0 (`--boot`/INIT2 gap vs a stubbed slot-compute op). Same class as the known
-   `G[0x624xx]` gfx boot-state gap. NOT a compositor/z-order/blend bug.
+   screen. ⇒ The layering failure is a **slot-assignment** problem. **CONFIRMED CAUSE (2026-07-08, live
+   `AGE_DIAG_SETTEX` trace):** every `set-texture` slot = `G[0x62452]`, written by `query-gfx-object?`
+   (`0x215`) which returns **-1** for the (correctly-unregistered) CG/effect handles → the fallback at
+   SC0000 `label_12649` does `G[0x62452] = lookup-array-2d(rec[s3]=G[0x3239], G[0x62450], 3, 0)` = **0**
+   because the slot table `rec[s3]`/`G[0x3239]` is **empty**. That table is filled by `call label_125bd`
+   (SC0000 `0x50f`, slots 4..13), which is reached **only through the scene-coroutine framework** — the
+   `G[0xaba5c]` gate (`0x450`) + op `0x140` (`u0041F9C0`, coroutine LABEL/yield `"LABEL" "J"` @ `0x46d`).
+   **Op `0x140` is stubbed** (SC0000 GAP list) → the coroutine re-entry never routes through `label_125bd`
+   → slot table stays 0 → all layers collapse into slot 0. This is exactly the "second still-latent gap"
+   flagged above (§"the render drift's second half"), now confirmed as the cause of the visible grey BG on
+   multi-object pages. **Fix = implement the scene-coroutine framework (`0x7b`/`0x7c`/`0x140` + `G[0xaba5c]`
+   gate) so `label_125bd` runs** (or, as a targeted unblock, run `label_125bd`/seed `G[0x3239]` directly).
+   NOT a compositor/z-order/blend bug. Diagnostics: `AGE_DIAG_SETTEX=1` env → VM logs each `set-texture`
+   slot operand + `query-gfx-object?` result.
 
 ---
 

engine/Age.Engine/Model/GfxState.cs

@@ -113,6 +113,7 @@ public sealed class GfxState
     /// <summary>Op 0x215 (query-gfx-object): native returns std::map::find(handle) — the registered value (=handle),
     /// or 0xffffffff (=-1) when the handle was never 0x1a2-registered. NOT a fabricated slot allocator.</summary>
     public int QuerySlot(long handle) => _registry.Contains(handle) ? (int)handle : -1;
+    public bool IsRegistered(long handle) { lock (_lock) { return _registry.Contains(handle); } }
     public long QueryField(long idx) => _fieldTable.TryGetValue(idx, out var v) ? v : 0;
 
     public void Release(long handle)

engine/Age.Engine/Vm/VirtualMachine.cs

@@ -16,6 +16,7 @@ public sealed class VirtualMachine
     private readonly IHost _host;
     private readonly VmOptions _o;
     private readonly IScriptProvider? _provider;
+    private static readonly bool _diagSetTexture = System.Environment.GetEnvironmentVariable("AGE_DIAG_SETTEX") == "1";
     private ExecFrame _cur = null!;
     private int _depth;
     private readonly ITraceSink _sink;
@@ -221,6 +222,9 @@ public sealed class VirtualMachine
                 Gfx.ClearSurface((int)Read(a[0]));
                 _host.CreateTexture((int)Read(a[0]), (int)Read(a[1]), (int)Read(a[2])); return pc + 1;
             case "set-texture":   // 0x1f9 (resId)(slot)(colorkey) — load a file into the slot's surface
+                if (_diagSetTexture)   // AGE_DIAG_SETTEX: log the SLOT operand source (literal vs which global) — grey-BG slot dig
+                    System.Console.Error.WriteLine($"[settex] resId=0x{Read(a[0]):x} slot={(int)Read(a[1])} " +
+                        $"slotOp=(type={a[1].Type} val=0x{a[1].Value:x}){(a[1].Type == 3 ? $" G[0x{a[1].Value:x}]" : "")}");
                 Gfx.SetSurface((int)Read(a[1]), Read(a[0]), a.Count > 2 ? Read(a[2]) : 0);
                 _host.SetTexture(Read(a[0]), (int)Read(a[1])); return pc + 1;   // host still tracks dims for get-texture-size
             case "draw-texture":   // 0x1fb (handle)(slot)(srcX)(srcY)(w)(h)(dstX)(dstY) — bind object -> surface + rect + pos
@@ -238,6 +242,12 @@ public sealed class VirtualMachine
             case "play-voice": _host.PlayVoice(Read(a[0])); return pc + 1;
             // ---- gfx command-buffer ops (VM-internal GfxState; docs/engine-re.md op-contract table) ----
             case "query-gfx-object?":   // 0x215 (out)(handle) -> slot | -1
+                if (_diagSetTexture)   // reuse the flag: show what the slot query returns (grey-BG slot dig)
+                {
+                    long h = Read(a[1]);
+                    System.Console.Error.WriteLine($"[query] handle=0x{h:x} handleOp=(type={a[1].Type} val=0x{a[1].Value:x}) " +
+                        $"-> QuerySlot={Gfx.QuerySlot(h)} registered={Gfx.IsRegistered(h)}");
+                }
                 Write(a[0], Gfx.QuerySlot(Read(a[1]))); return pc + 1;
             case "query-gfx-field?":    // 0x216 (out)(idx)
                 Write(a[0], Gfx.QueryField(Read(a[1]))); return pc + 1;