Print emulation: WebGL2 ink bleed post-processing filter

Integrate the ink-bleed sketch as a rendering option. InkBleedRenderer class
wraps a WebGL2 pipeline that post-processes the score canvas: separable
Gaussian blur + composite shader with domain-warped ink bleed, FBM edge
roughness, letterpress pooling, paper fiber grain, micro-spatter, and warm
paper color. Toggled via 'Print' button in toolbar. Lazy-initialized on
first toggle. Default preset: subtle vintage printed score look.

Based on ~/explorations/random-sketches/vibes/ink-bleed.

Author: zz85

index.html

@@ -330,6 +330,7 @@
 							oninput="applyZoom(parseFloat(this.value))"
 							style="width: 80px;" />
 						<span id="zoom_label" style="font-size: 11px; min-width: 36px; text-align: center; color: var(--text-dim);">100%</span>
+						<button id="ink_bleed_toggle" title="Print emulation: ink bleed, paper grain, letterpress effect">Print</button>
 					</div>
 
 					<!-- Spacing tuning (popover) -->