feat(text): retain bounded raster capacity

[thejustinwalsh]

Summary

• add deterministic 25% instance-capacity slack, capped at 256 glyphs, to Bitmap, MTSDF, and Slug batches;
• retain batch, geometry, material, texture, attribute, and backing-array identity for arbitrary glyph replacement that fits compatible capacity and page-run topology;
• publish authoritative shrink and exact-capacity draw counts, with transactional replacement for overflow or incompatible Bitmap/Slug page runs;
• coalesce dirty uploads through shared 32-instance buckets, fall back after eight fragmented ranges, and carry renderer-pending ranges into later stages;
• make React conformance inspect submitted logical instances rather than unused retained-capacity slots;
• add focused lifecycle regressions, exact package-size evidence, and canonical Milestone 10 documentation.

This is roadmap item 10.3 and is stacked on #13. It does not add renderer-wide batching across independent Text objects.

Verification

• pnpm --filter @pmndrs/text test — 191 package/integration tests and 6 deterministic fuzz-smoke tests passed;
• focused retained-capacity suite — Bitmap, MTSDF, and Slug replacement, shrink, exact-capacity growth, abort, overflow disposal, page-run topology, object/buffer identity, and color-only preservation passed;
• pnpm --filter @pmndrs/text-benchmarks test:unit — 261 tests passed;
• pnpm --filter @pmndrs/text-benchmarks test:headless — all 19 conformance cases pass, including React reconciliation with retained capacity;
• pnpm --filter @pmndrs/text-benchmarks check:size — exact report and reviewed ceilings passed;
• pnpm --filter @pmndrs/text-benchmarks check:autoresearch-baseline passed;
• repository typecheck, lint, format, and pnpm docs:check passed with zero OKF errors or warnings.

Browser core, baker hosts, and Wasm artifacts remain byte-identical. Relative to #13, optional Bitmap, MTSDF, and Slug closures grow by 598, 762, and 1,204 Brotli bytes respectively and remain under existing absolute budgets.