[Mistium]

Two crash paths in the renderer and one diagnostic.

WebGL context loss. Nothing listened for webglcontextlost, so when the browser took the context away (GPU reset, too many WebGL tabs, a backgrounded mobile tab) the next draw() failed inside _buildShader with an unhelpful compile error, and because nothing called preventDefault the browser never tried to restore the context. The renderer now:

• listens for both events on its canvas, calls preventDefault on loss, sets isContextLost, skips draw() while lost, and emits ContextLost;
• on restore, rebuilds the shader cache, geometry, and draw region state, asks every skin to drop the textures that died with the old context, marks itself dirty, and emits ContextRestored.

Each skin gets an onContextRestored hook. SVG skins rebuild their MIPs from the still-loaded image on the next draw, text bubbles re-render from their text, the pen layer recreates its shaders, buffers, and framebuffer (its contents are gone, as they would be in any renderer), and bitmap skins stay empty until the VM sets them again. The VM and GUI can listen for the two events to reload costumes or tell the user; this PR keeps the renderer usable and quiet instead of throwing every frame.

Sparse draw list. With the experimental real layer indexes setting enabled, setDrawableOrder leaves holes in _drawList, and shifting entries turns holes into explicit undefined. _drawThese and _candidatesTouching already skip those, but pick() did not, so clicking the stage threw on the first hole. It now skips holes and destroyed ids.

Shader diagnostics. Production has reported ERROR: 0:1: 'vertex' : syntax error, which cannot come from sprite.vert itself: line 1 of every compiled shader is a #define. The compile failure now includes the first 80 characters of the vertex and fragment source that were actually compiled, so the next report shows what the bundler handed over.

Also fixes the lint errors in test/unit/ResourceDisposalTests.js so npm test no longer fails at the lint step.

Tests: eslint . clean, tap test/unit passes (new ContextLossTests.js covers the lost and restored paths and the sparse pick), and npm run build && tap test/integration passes (pick, scratch, skin size).

No visible surface.