site(math): joy.html — showcase v0.3 grammar with byte-locked SVG previews (#68)

## Summary

Updates `site/math/joy.html` to highlight the four-RFC + three-v2
grammar expansion that shipped in v0.3 (`recurrence` / `geodesic` /
`pde-solve` / `streamline.colorBy`). The page previously claimed Glyph
"doesn't model these yet" — that was true when it was first written but
isn't true any more.

## What's new

- **Hero copy rewritten** — emphasises that every demo is now backed by
a Glyph spec.
- **v2 grammar banner** — slim ribbon under the hero with four pills
jumping to the demos where each new shape/mark is showcased.
- **Per-demo Glyph spec preview** — each of the 9 demos now has a
`.glyph-output` panel beneath the info column showing the byte-locked
SVG the Glyph compiler produces for the same math. Demos that cover both
v1 and v2 (gravity-lens, flow-field, waves) get two panels so the v1→v2
expansion is visible at a glance.
- **Grammar Catalog section** — new bottom-of-page showcase with 5 cards
(trajectory, recurrence, geodesic, pde-solve, streamline) each pairing a
JSON snippet with the rendered SVG side by side.
- **"What this means for Glyph" rewritten** — celebrates the shipped
grammar instead of describing it as future work.

Eleven fixture SVGs copied into `site/math/` for serving. All remain
byte-locked in CI via `__fixtures__/math/*.test.ts`; this is a static
copy for the site.

Page weight grows from ~25 KB to ~49 KB; per-demo SVGs use
`loading="lazy"` so initial paint stays fast.

## Test plan

- [x] Manually verified all SVG references resolve to local files in
`site/math/`
- [x] HTML tag balance check (12/12 sections, 95/95 divs)
- [x] Anchor targets verified (#grammar-catalog, #curlicue,
#gravity-lens, #waves, #flow-field, #lissajous)
- [x] Core test suite unchanged (658/658)
- [x] Lint clean (no new warnings)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: seanhanca <infraservice@livepeer.org>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: 3 people