feat(simulation): lake_margin_q depth band — lake shorelines gain gradient vocabulary (T-1188)

Lake edges rendered as hard step-edges while ocean coasts got multi-tone
transition bands: every coastal-transition morphology gate keys on
ocean_fraction_q, definitionally 0 inside a lake basin (hypothesis (b)
of the ticket; (a) disproven first — a shoreline-crossing sweep at
2048/512/128m plus a 10m fine sweep all land on the same continuous
crossing, so positional refinement was never broken). New
DistrictProfile.lake_margin_q (0-100 settled-hydrology depth band, from
the same bilinear filled/elevation pair the lake test already samples;
ceiling calibrated just above the observed p90 depth on GJ338Bd's 5,043
flooded cells), threaded through both derive paths onto
EncodedStepCanvas (serde-default for shape tolerance) and down the
client: protocol decode, terrain-layer plane, colorize shades Lake cells
by depth band instead of elev_q (bedrock-under-water, the wrong signal).
project.yaml 0.4.0 -> 0.4.1: the new wire field must invalidate the
client disk cache via its version tag (T-1183's D-192 mechanism).
Acceptance gates green with the new field (lossless round-trip,
cache-hit==cache-miss, every rung).

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

project.yaml

@@ -1,7 +1,7 @@
 name: The Settled Reach
 # Version scheme: 0.{phase}.{n} — phase = active Development Cascade phase (D-166).
 # Phase 4 (deterministic world generation) is active.
-version: 0.4.0
+version: 0.4.1
 repository: settled-reach