feat: add MasonryLegendList

[maxwellyoung]

Fixes #23.

What this adds

• MasonryLegendList from the separate @legendapp/list/masonry entrypoint
• deterministic shortest-column placement for fixed and dynamically measured item sizes
• gap-aware balancing, recycling, data updates, column-count changes, and total-size updates
• native and web fixture routes plus README/changelog documentation
• a small internal layout-strategy hook so masonry reuses LegendList's existing container pool, measurements, viewability, MVCP, and imperative scrolling paths

The initial API is deliberately vertical-only. Column spans and overrideItemLayout are not supported.

Bundle isolation

The masonry algorithm is only present in the optional integration bundle:

• masonry.mjs: 4.8 KB
• masonry.js: 5.4 KB

It imports @legendapp/list/react-native rather than repackaging core. The postbuild integrity check now covers the masonry entrypoint, and the publish dry run includes masonry.d.ts, masonry.js, and masonry.mjs.

Reproduce

Web:

bun install
cd example-web
bun install
bun run dev:fixtures

Then open /masonry. The fixture uses 80 dynamically measured cards and recycling across three columns.

Native:

cd example
bun install
bun run ios:fixtures

Open Data & Layout > Masonry.

Verification

• bun test: 1,519 passed, 0 failed
• bun run tsc: source, native example, and web example passed
• bun run build: Biome, tsup, declarations, and postbuild integrity passed
• exact 10,000-item test validates every position/column, total content size, and one fixed-size lookup per item
• Playwright smoke at 1280x900 and 768x900: no console errors or warnings; scrolling recycled the rendered pool from cards 1-22 to cards 41-63
• Expo/Metro iOS fixture export: 1,881 modules bundled successfully into an 8.29 MB Hermes bundle, including the @legendapp/list/masonry native fixture route

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