fix(simulation): derive at the octaves a rung can actually reconstruct (T-1240)

Region rendered as fine uniform stucco while District and Quarter, on identical
code, read as terrain. The cause was sampling: `min_wl_m` arrives as an LOD
request and defaults to 0, so every invented octave contributed at every rung.
MIN_WL_BANDS_M was meant to be the floor but is built from the rung's CELL SIZE
(2 x DISTRICT_M), which stopped being the sample spacing at the D-255 extent
inversion — a rung fixes EXTENT now and spacing falls out of the canvas size.
The bands were off by roughly the cell count, and the served path never consulted
them anyway.

The cutoff is now derived from the resolved spacing, which is what this ticket
asked for. Two things had to be measured rather than reasoned to get it right,
and both corrected me.

FIRST: the field was the culprit, not the renderer. I attributed the stucco to
the client stipple painting noise onto a smooth field. Surfacing the terrain
layer's own mean |relief_q gradient| in the capture readout settled it in one
shot: Region 18.24 steps per cell — 144 m of relief between NEIGHBOURING cells —
against District's 0.30 and Quarter's 0.07. The server was sending noise. That
diagnostic ships here for the same reason `plane_variety` did in T-1213: a noisy
field and a renderer inventing noise look identical, and one number separates
them.

SECOND: Nyquist is the wrong threshold. The first version floored at 2 x spacing,
the aliasing limit, and Region barely moved (56.16 -> 59.73 lum spread, gradient
still 18.24) because 2 samples per cycle is unaliased but renders jagged. The
rungs that already worked say what the real bar is: District reconstructs its
finest surviving octave at 34 samples per cycle, Quarter at 135. At 8x, Region
goes to 1.08 gradient and 70.01 spread, and shows ridges and valleys.

THE TRADE, taken deliberately and recorded in the tests: an 8x floor also
truncates the coast warp's 2,048 and 1,024 m octaves at Region, the band T-1160
added for "one coastline at every rung". An earlier test here asserted that band
must survive; it now asserts the opposite. Same reasoning as the relief: a
1,024 m coastline wiggle at 379.3 m per cell is 2.7 samples per cycle, so drawing
it draws noise rather than coastline character — a rung cannot show shape finer
than its own cell. The warp is amplitude-capped sub-pixel on the working grid, so
what is lost is small. If a future pass wants the warp exempt, the fix is a
relief-only floor threaded through derive_at_metres, NOT a lower multiple, which
takes the stucco back.

Global is exempt: its floor would be ~70 km and would truncate the whole warp
band, and it needs none — the orbital derive leaves relief_q flat at 50. District
(3.79 m spacing) and Quarter (0.948 m) floor below every octave in play and
derive byte-identically, which their own test pins.

Cache-safe by construction: the floor is a pure function of (rung, extent,
body_radius), all three already in the step-canvas cache key. 0.4.12 is required
anyway — this changes derived BYTES at Region, so a 0.4.11 entry holds a field
this build would never produce.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

project.yaml

@@ -61,7 +61,11 @@ name: The Settled Reach
 # 0.4.11 — per-class cover marks (copse, thicket, rocky outcrop). Paint again,
 # not derivation; bumped because the client step_canvas cluster is in the T-1242
 # registry.
-version: 0.4.11
+# 0.4.12 changes DERIVED BYTES, not paint (T-1240): Region canvases now truncate
+# every octave the rung cannot reconstruct, so a 0.4.11 Region entry holds a
+# relief field this build would never produce. Warm caches are genuinely wrong
+# here, not merely stale.
+version: 0.4.12
 repository: settled-reach