fix(webgl): fill entire quad index buffer; cap at Uint16 limit

[chiefcll]

What

createIndexBuffer only filled valid indices for ~1/6 of the quad index buffer, silently capping renderable geometry at ~4167 quads per frame. Anything beyond that drew with all-zero indices (degenerate triangles) and rendered nothing.

Root cause

const indices = new Uint16Array(maxQuads * 6); // 150000 slots
for (let i = 0, j = 0; i < maxQuads; i += 6, j += 4) { ... }

i is the write cursor into the index array and advances by 6 per quad (six indices per quad), so the bound must be maxQuads * 6. Stopping at i < maxQuads exited ~6× too early, leaving ~5/6 of the buffer at its initialized value of 0. A quad whose six indices are all 0 is two zero-area triangles over vertex 0 → invisible.

Because SDF glyphs index into this same shared buffer, text added last in a frame (e.g. a high-zIndex debug overlay drawn over a large card grid) fell into the dead zone and disappeared, while everything before it rendered correctly.

Fix

• Loop bound is now i < maxQuads * 6, so every slot receives valid indices.
• maxQuads is capped at 16384. Uint16 element indices can only address 65536 vertices (16384 quads × 4); the previous value of 25000 would have overflowed the vertex id past 65535 and wrapped to the wrong vertices. The capped index buffer is also smaller on the GPU (192 KB vs ~293 KB) while raising usable capacity ~4× (4167 → 16384 quads).

Impact

This affected any scene exceeding ~4167 quads in a frame — not just text. Large grids (e.g. viewport-memory) were hitting it. Reproducible with the stress-tv example at tier 4 / 200 cards + ?debug=true: the overlay truncated mid-line; with this fix it renders fully.

Tests

• Adds RendererUtils.test.ts: asserts the buffer is fully populated (including the last quad, which the old loop left zeroed) and that maxQuads caps at 16384 with the final vertex id landing exactly at 65535.
• Full unit suite passes (318/318).

Notes for reviewer

• The 16384 cap is the hard ceiling of Uint16 element indices. Raising bufferMemory past 16384 * 80 no longer grows this buffer; going beyond would require 32-bit indices (OES_element_index_uint / WebGL2).
• WebGL-only path; no Canvas backend equivalent.

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