Track structured GLSL diagnostics in playground error reports

[devallibus]

Follow-up from PR #75 review.

Context:

• The current playground error pipeline preserves structured error kinds for TSL, but GLSL still collapses diagnostics to plain string arrays in the browser-facing report path.
• apps/web/src/lib/webgl-shader-errors.ts already emits ShaderDiagnostic entries with kind values that map directly onto PlaygroundError.

Goal:

• Preserve GLSL structured diagnostics end-to-end so the playground and API can report glsl-compile and glsl-link through PlaygroundErrorReport, not only raw message strings.

Suggested scope:

• Add a conversion step from ShaderDiagnostic[] to PlaygroundError[] near the existing GLSL error reporting path.
• Update PlaygroundCanvas to send structured GLSL errors alongside plain error strings.
• Keep existing plain-message behavior for UI compatibility while enriching the structured payload.
• Add tests covering both compile and link diagnostics flowing through the error report layer.

Acceptance criteria:

• GLSL compile failures surface at least one structured error with kind glsl-compile.
• GLSL link failures surface at least one structured error with kind glsl-link.
• Existing consumers of report.errors continue to receive readable messages.
• Tests cover the structured GLSL fallback/reporting path.