feat(text): add MSDF font atlas text rendering module

[stormmuller]

Summary

Adds a new @forge-game-engine/forge/text module implementing the MSDF (multi-channel signed distance field) text rendering path proposed in #584:

• FontAtlas type + loadFontAtlas (/asset-loading) - parses an msdf-atlas-gen JSON metrics file and loads its atlas PNG (via the existing ImageCache) into glyph advances, kerning pairs, and line metrics, all normalized to em units. Kept data-only (no WebGL) so /asset-loading keeps its existing zero-dependency-on-rendering direction.
• MSDF fragment shader + createMsdfTextRenderable (/rendering) - reuses the existing sprite.vert shader and its instance data layout (spriteInstanceDataSegment) completely unchanged, paired with a new msdf.frag shader (median-of-three-channels reconstruction + fwidth-based screen-space-derivative antialiasing, the standard technique from msdf-atlas-gen's own docs). One renderable is built per FontAtlas and shared across every entity using that font, exactly like a sprite's Renderable.
• TextEcsComponent/addTextComponent, TextMeshEcsComponent, and createTextShapingEcsSystem (/text) - shapeText (pure, fully unit-tested) word-wraps, applies kerning, and lays out lines by alignment/line-spacing/pivot into glyph quads. The shaping system is dirty-tracked: it only re-shapes an entity when text, font, fontSize, wrapWidth, lineSpacing, alignment, or pivot actually changed, not every frame.
• createRenderEcsSystem text integration - per the issue's own risk analysis, this skips the larger "generalize nine-slice into a shared SubQuad[] expansion" refactor in favor of the issue's suggested fallback: an isolated pushTextRenderCommands/buildTextCameraCommands pair that turns each glyph into a SpriteEcsComponent-shaped render command. This reuses the sprite instancing/batching/depth-sort pipeline completely unchanged (zero modifications to the existing sprite/nine-slice code path) while still letting text interleave correctly with sprites in one sorted command buffer.

Not included (follow-up work)

Flagged explicitly in the new docs' "Current limitations" section:

• No font shipped with the engine - generating a real MSDF atlas requires the msdf-atlas-gen toolchain, which isn't available in this environment. Every project currently supplies its own atlas.
• No Canvas2D prototyping escape hatch - the issue proposes a documented fillText-to-texture fallback for quick prototyping; not built here.
• No text effects (outline/glow/shadow) - these fall out as extra MSDF shader parameters later.
• No demo - without a real generated font atlas to point one at, a /documentation-site/src/pages/demos/text demo would have nothing genuine to render. Everything else in the module (shaping math, dirty tracking, shader, render-system integration) has full unit-test coverage instead.

Related issue(s)

Closes #584

Verification checklist

• npm run check-types passes with 0 errors
• npm test passes (1084 tests, 161 files)
• npm run lint passes with 0 errors
• npm run cspell passes with 0 errors
• npm run check-exports passes (@forge-game-engine/forge/text resolves 🟢 ESM/bundler like every other subpath)
• New public API is exported from each module's index.ts, /src/index.ts, and package.json exports (new ./text entry)
• Documentation added under /documentation-site/docs/docs/text (overview + an MSDF Text guide covering atlas generation, wrapping/alignment, registration order, and dirty tracking), plus updates to the asset-loading and rendering section docs to cross-link it
• No existing demo touches TextEcsComponent/the new msdf shader, so none needed updating - but since this PR does touch /rendering (render-system.ts, the shader cache, shader index), I still ran npm run build, documentation-site's npm run typecheck/npm run build, and loaded the Nine-Slice demo (the closest existing exerciser of the sprite render-command path this PR adds a sibling branch next to) plus the new /docs/text and /docs/text/msdf-text pages in a real browser (Playwright/Chromium) to confirm no regressions and no MDX/build errors

Changelog

• Added a bullet under ## [Unreleased] → #### Added in CHANGELOG.md

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Generated by Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 96.40288% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/asset-loading/load-font-atlas.ts	90.00%	0 Missing and 2 partials ⚠️
src/rendering/systems/render-system.ts	91.30%	0 Missing and 2 partials ⚠️
src/text/utilities/shape-text.ts	98.30%	0 Missing and 1 partial ⚠️

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