fix(shader): emit contextLost when shader creation fails on a lost GL context

[chiefcll]

Problem

Production telemetry on modern Chrome/Edge surfaced:

Unable to create the shader: VERTEX_SHADER. WebGlContext Error: 37442

37442 = 0x9242 = CONTEXT_LOST_WEBGL. The GL context was lost, so gl.createShader() returned null and WebGlShaderProgram threw. Two facts make this nasty:

• Shader programs compile lazily, once per distinct shader cache key, and the cache is never cleared. So this isn't startup-only — it fires the first time any not-yet-compiled shader variant is reached, which can be deep into a session.
• Shader creation runs on the consumer's reactive stack (convertToShader → renderer.createShader()), which can execute before the async webglcontextlost DOM event is processed. The throw then propagates out of the public API with no contextLost signal, so consumers have nothing to react to and the exception tears through their reactive layer.

Fix

Detect the lost context at the point of failure and route it through the existing contextLost channel, splitting responsibility cleanly:

• WebGlContextWrapper — add an isContextLost() passthrough. getError() can't be reused for detection here: its CONTEXT_LOST_WEBGL flag is cleared on first read (already consumed building the error message). isContextLost() is stateful and reliable.
• WebGlRenderer.createShaderProgram (backend-specific) — wrap in try/catch; on a genuinely lost context, trip stage.setContextLost() (idempotent; emits contextLost), then rethrow. Closes the race with the async webglcontextlost event.
• CoreShaderManager.createShader (backend-agnostic) — on a lost context, fail soft by returning the default shader node so the throw doesn't propagate through the consumer's reactive layer (recovery is an app reload, per BROWSERS.md). A genuine GLSL compile error (context not lost) still rethrows loudly so real shader bugs aren't masked.

The try/catch sits on the cache-miss path only (once per shader variant), not a hot path.

Behavior note

On context loss this fails soft rather than rethrowing, since the context is unrecoverable in-place and the render loop has already stopped. If a rethrow (e.g. to hit a SolidJS <ErrorBoundary>) is preferred, the CoreShaderManager branch is a one-line flip.

Consumers still need a contextLost listener to actually recover (prompt/auto reload) — the engine deliberately does not rebuild GPU resources in place.

Tests

• CoreShaderManager.contextLoss.test.ts — fail-soft returns default node on loss; rethrows on genuine compile error; failures aren't cached.
• WebGlRenderer.contextLoss.test.ts — reproduces the exact Unable to create the shader: VERTEX_SHADER. WebGlContext Error: 37442 throw; verifies setContextLost() fires only on a lost context.

Full suite: 251 passing. Build + lint clean.

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