docs(examples): immediate-mode rendering example + guide (A5)

[Exoridus]

What

Adds the missing Immediate-Render example + guide for v0.14 Phase 4D (PR-3 / Backlog A5). The drawGeometry / RenderBatch + drawBatch API shipped in #150/#151 but had no example or guide page; this closes that gap.

Example — geometry-graphics/immediate-mode-rendering

A node-free, procedural scene:

• drawGeometry — 14 procedural gears (regular polygons built as interleaved position + color geometry), each drawn individually with its own raw world Matrix and tint. One draw call each.
• RenderBatch + drawBatch — a field of 2,400 sparks of one shared static geometry, rebuilt each frame and submitted as a single instanced draw call.
• Live overlay (mountControls) shows context.stats.drawCalls, and a mountControlPanel toggle flips the spark field between the instanced batch and one drawGeometry per spark, so the draw-call count visibly jumps from a handful to thousands — proving the batch is one draw call.

Guide — rendering/immediate-mode.mdx

New advanced chapter in the Rendering part: when to use immediate mode vs. the retained scene graph, building standard-layout geometry, drawGeometry, RenderBatch/drawBatch, and reading RenderStats.drawCalls. Embeds the example via <ExamplePreview>. Registered in guide-structure.ts with examples + apiLinks (rendering-context, render-batch, geometry, matrix, color).

Verification (all green locally)

• pnpm typecheck:examples ✅
• pnpm typecheck:guides ✅
• pnpm lint ✅ (0 errors; 4 pre-existing warnings in input/keyboard.js)
• pnpm format:check ✅
• guide-structure / example-search / guide-prose-links / playground-nav tests ✅ (65 tests)
• examples:sync-generated .js committed; examples.json entry added by hand
• Construction sanity-checked against real source (RenderBatch static-geometry guard, add/clear/count, polygon build) via throwaway vitest (removed)

Notes

• No src/ changes — examples + docs only.
• backend: "core" (uses the backend-agnostic RenderingContext immediate path).