feat(simulation): relief_q — a local relief signal the deep rungs can resolve (T-1213)

The District and Quarter rungs rendered as flat colour, and the cause was not
the biome work everyone assumed. Measured on Ferrath through the production
canvas builder: at District the mean |elev_q delta| between neighbouring
gridunits is 0.02, and NOT ONE PAIR in a 1290x540 frame differs by 2.

elev_q spans 0-100 across the body's whole 8 km elevation range, so ONE STEP IS
80 METRES. A District canvas covers 2,048 m of ground, where the rolling relief
a walker navigates by is metres to tens of metres -- a fraction of a single
step. The sub-district detail IS generated (invent_primitives' scatter and
relief bands compute it) and then rounded away. Confirmed by running the
diagnostic with the octave cutoff disabled: still 0.02.

relief_q carries that same invented fine component against a scale chosen to
resolve it: 0-100 about a flat 50, RELIEF_FULL_SCALE_M = 400 m either side, so
8 m per step -- ten times finer than elev_q. elev_q keeps its body-absolute
meaning and the Atlas legend stays true.

Measured effect, elev_q vs relief_q (distinct values / mean 4-cell delta):

  Region     49 / 2.38   ->   101 / 21.86
  District   10 / 0.08   ->    35 / 0.35
  Quarter     8 / 0.02   ->    19 / 0.06

FIXED metre scale, never per-canvas normalization: the value for a piece of
ground must not depend on what else is in frame, or the same hillside changes
tone as the viewer pans. And it excludes elev_pct deliberately -- this is the
departure from the surrounding land, not height above sea level; including the
base would re-introduce the body-scale dominance that makes elev_q unusable
down here.

50 at the orbital rungs, which skip invent_primitives by design. Nothing is
lost: Global and Region still have varied elev_q (101 and 49 distinct values),
and the client takes whichever field carries signal via a max, with no
rung-name branching.

The client's ruggedness driver changes with it. It was an elev_q GRADIENT,
which cannot work across rungs -- the same 4-cell delta reads 21.86 at Region
and 0.08 at District, so any single full-scale constant either saturates one or
vanishes on the other. relief_q states relief outright, so |relief_q - 50| is
the answer directly and a fixed metre scale is immune to that by construction.

An absent plane reads FLAT, not zero -- 0 on this field means maximum relief
BELOW flat, so a payload without it would have stippled the entire map. That is
reachable: the field is #[serde(default)] so old-shape payloads decode. Two
colorize tests whose fixtures predate the plane caught it.

2004 server tests, 1838 client tests, 0 failed. clippy clean.

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

server/tests/derivation_harness.rs

@@ -2268,6 +2268,7 @@ fn make_region(
         precipitation_class: precip,
         slope_q,
         elev_q,
+        relief_q: 50,
         ocean_fraction_q,
         lake_margin_q: 0,
         river_threshold: derive_river_threshold(tectonic, precip),