…ines

A new <Hexbin> Svelte component that bins 2D scatter data into a
pixel-space hex lattice, runs reducers per bin (default fill='count'),
and renders each non-empty bin as a regular hexagonal cell. Cells are
regular by construction because the lattice is computed in pixel space
(after scales exist) rather than in data space - they tile correctly
under any axis aspect ratio. Pairs with the existing <Hexgrid> at
matching binWidth: a default <Hexbin/> + default <Hexgrid/> tile the
same lattice with no user coordination.

Architecture context: see HEXBIN_DESIGN_COMPARISON.md (local). This is
Approach B in that doc - the component-flavored alternative to the
existing transform-flavored hexbin(), which stays in place for now.

Shared lattice helper
- packages/svelteplot/src/helpers/hexLattice.ts (new, 102 lines):
  hexLattice / hexLatticeXY / hexCenter / pointToHex / hexagonSubpath /
  hexCellsInRect. Single source of truth for hex math used by both the
  transform and the components.
- packages/svelteplot/src/helpers/hexLattice.test.ts (new, 7 tests).
- transforms/hexbin.ts refactored 176 -> 142 lines to use the helper.
- marks/Hexgrid.svelte refactored 92 -> 73 lines to use the helper.

Bug fixes
- Color scale: reduceOutputs writes to __fill/__stroke (with __ prefix,
  see helpers/reduce.ts:113), not fill/stroke. The Symbol-copy step in
  Hexbin.svelte was reading the wrong key, so FILL_VAL stayed undefined
  and cells fell through to plot.options.color.unknown (#cccccc99).
- Y-origin alignment: Hexbin called hexLattice(bw, ml+bw/2, mt+bw/sqrt(3))
  while Hexgrid (Hexgrid.svelte:53) and the existing transform
  (transforms/hexbin.ts:85) called it with originY = mt. Hexbin was the
  outlier - test svelteplot#6 caught the offset (= binWidth/sqrt(3) = 11.547 px).

NEW_MARK_CHECKLIST (rebased to monorepo paths from main@04668ed4):

Source & Types
- [x] Component source             packages/svelteplot/src/marks/Hexbin.svelte
- [x] Props interface w/ generics  Hexbin.svelte:16-43
- [x] JSDoc @component             Hexbin.svelte:1-15
- [x] MarkType addition            packages/svelteplot/src/types/mark.ts
- [x] PlotDefaults type entry      packages/svelteplot/src/types/plot.ts:341
- [x] PlotDefaults hook call       Hexbin.svelte:47, 76-78

Exports
- [x] Barrel export                packages/svelteplot/src/marks/index.ts:34
- [x] Top-level re-export          auto via packages/svelteplot/src/index.ts
- [x] Package exports              auto via packages/svelteplot/package.json:43

Documentation
- [x] Docs page                    src/routes/marks/hexbin/+page.md
- [x] Docs data loader             src/routes/marks/hexbin/+page.ts
- [x] Live code examples           three in +page.md
- [x] Sidebar entry                config/sidebar.ts
- [x] API reference regenerated    src/routes/api/marks/+page.md:541
- [x] API anchor link              src/routes/api/marks/+page.md:7

Testing
- [x] Unit test fixture            packages/svelteplot/tests/hexbin.test.svelte
- [x] Unit test driver             packages/svelteplot/tests/hexbin.test.svelte.ts
- [x] Coverage: rendering, custom class, empty data, single-bin (count
      reducer plumbing), invalid coords, accessor functions, regular
      hexes under 2:1 aspect, binWidth scaling, Hexbin/Hexgrid alignment

Examples & Visual Regression
- [x] Examples gallery             src/routes/examples/hexbin/
                                     0-basic.svelte
                                     1-with-hexgrid.svelte
                                     2-custom-binwidth.svelte
- [x] Frontmatter (title/description/data/sortKey) in each
- [x] VR baselines (light + dark)  src/snapshots/hexbin/
                                     0-basic{,.dark}.png
                                     1-with-hexgrid{,.dark}.png
                                     2-custom-binwidth{,.dark}.png

Tooling fix (incidental, pre-existing breakage)
- scripts/generate-api.js: 13 stale src/lib/... paths updated to
  packages/svelteplot/src/... - leftover from the monorepo refactor
  in commit 3a1d520.

Tests: 870/870 pass. pnpm check: 0 errors.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>