feat: add calendar component

[himself65]

Summary

Adds a calendar component to the catalog — a day-grid month picker modeled on shadcn's base calendar. It ships both ways, matching the stepper/chart path (a composite that lowers to primitives — no bespoke widget kind, no golden pixel signatures):

• <calendar> in Native markup, wired through both lowering backends (interpreter + compiled engine)
• Ui.calendar in the Zig builder, taking a typed CalendarSelection union

Display-only like the rest of the library: the app model owns the shown month and the selection. Day presses inject the pressed CalendarDate into a bare on-select Msg; the chevrons dispatch on-prev/on-next so update steps the month via CalendarDate.addMonths.

What's included

• calendar_date.zig — a CalendarDate value type: leap-year math, Sakamoto weekday, Hinnant civil-from-days arithmetic, ISO YYYY-MM-DD parse/format, and month-grid helpers (gridStart, addMonths, …). 11 standalone unit tests.
• Selection modes — single, range (inclusive start/end band), and multiple. Today gets a soft fill, the selected day the accent fill, adjacent-month days read muted, and min/max render out-of-bounds days disabled.
• Full wiring — markup validator, binary serializer events table (select/prev/next), and the schema registry (stable codes + regenerated fingerprint pins).
• Docs — engine-rendered preview tiles (single hero + full grid), a live WASM preview scene, and a /components/calendar page (markup, Zig builder, attribute table). Index grid and component vocab updated.
• Changelog fragment under changelog.d/.

Testing

• zig build test — green (adds calendar_date, ui_tests, ui_markup_tests, ui_markup_view_tests cases covering the grid, nav, all three selection modes, disabled bounds, and outside-days).
• scripts/gate.sh fast — all suites PASS.
• pnpm --dir docs check — green; /components/calendar builds.

Notes

• Dates ride attributes as YYYY-MM-DD strings (literal or {binding}).
• mode picks how selected-dates reads: one date for single, a start/end pair for range, a comma/space-separated list for multiple.

🤖 Generated with Claude Code

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