feat(canvas): curated named GPU shader effects for scripted-UI canvas

[shipyard-local]

Add a SAFE, curated named-effect capability to the scripted-UI canvas so
generated / scripted plugin UIs (Forge) can apply signature GPU post-effects
by NAME — never by injecting arbitrary SkSL. Closes the retro/neon/tube/CRT
look gap that the CSS-filter path (blur/brightness/…) cannot cover.

New JS bridge entry, distinct from the CSS-filter path:
canvasSetShaderEffect(id, name, intensity=1.0)
Curated effects (each: single clamped intensity in [0,1]):
crt — scanlines + barrel curvature + vignette + aperture mask
grain — static film grain
vignette — radial edge darkening
noise — isotropic procedural value-noise overlay
brushed — horizontal brushed-metal streaks
bloom — threshold + small-kernel glow (neon/tube)
"none"/"" disables; an unknown name is a graceful no-op (never rejects a
build). intensity is clamped host-side.

Implementation:

• core/canvas/src/named_shader_effects.{hpp,cpp}: vetted SkSL library.
  Effects compile once and cache for process lifetime (RuntimeEffectCache),
  keyed by source; uniforms clamped; unknown name / SkSL compile failure
  both return a null filter.
• Canvas::save_layer_with_shader_effect(): new virtual. Default (non-Skia /
  CoreGraphics / RecordingCanvas) falls back to a plain save_layer so the
  effect is skipped and headless CPU renders never crash. SkiaCanvas builds
  an SkImageFilters::RuntimeShader over the compositing layer via push_layer.
• The RuntimeShader image filter runs in DEVICE space, so the effect gets
  the CTM-scaled resolution — vignette centering / CRT curvature / bloom
  radius stay correct under HiDPI (proven by a 2x-scale test).
• CanvasWidget::set_shader_effect() stores sticky, declarative effect state
  (persists across command-list rebuilds — no per-frame re-issue, no
  animation loop needed for a static overlay) and opens the existing
  per-canvas layer through the effect path when active — no extra layer.

Tests (Skia raster, headless): each named effect measurably changes pixels
vs. baseline; unknown name is a byte-identical no-op; intensity 0 is identity;
vignette stays device-centered under 2x scale; bloom bleeds light past bright
edges. Bridge API manifest + generated TS/docs regenerated.

Hotspot-Grow: core/view/src/widget_bridge/canvas2d_api.cpp reason="one bridge registration for the new canvasSetShaderEffect capability"
Hotspot-Grow: core/canvas/include/pulp/canvas/canvas.hpp reason="one virtual (save_layer_with_shader_effect) + its doc for the curated effect capability"
Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_015tUppDEybTgFDxS5BGKBWV

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🔎 Provenance

Agent	claude
Machine	m5
Tab	Analyze Pulp for AI plugin builder capabilities
Directory	~/Code/pulp-shader-fx
Session	14ef2f39-9113-4e61-bd8b-8faab97cab00

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claude --resume 14ef2f39-9113-4e61-bd8b-8faab97cab00

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cmux surface resume get --surface BD0A704A-23E0-4F9B-88CC-25BD1ACDAFEA

Restore URL — https://claude.ai/code/session_015tUppDEybTgFDxS5BGKBWV

_{stamped 2026-07-21 03:08 UTC}