Replaces the straight 2px diagnostic underline bar (PR #65) with an
actual sine squiggle. A dedicated SquiggleRenderer pipeline whose
fragment shader computes A·sin(x·2π/λ) and alphas pixels by distance
to the curve (fwidth/smoothstep — core WGSL, no MSAA or feature
flag). Geometry reuses push_glyph_extent_rects unchanged; only the
vertex format differs, adding a uv: absolute screen-x (so phase is
continuous across separately emitted glyph-run rects) and signed px
from the band centerline.
The diagnostic underline is split out of the solid wash batch into
its own buffer + draw, slotted under the glyphs where the bar was;
washes (selection / current-line) stay solid quads. The severity
palette (decoration_kind_to_underline_color) and minimap marks are
untouched. DIAG_UNDERLINE_PX (2px) → DIAG_SQUIGGLE_PX (6px) so the
wave fits inside the band.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
