feat: add Panda CSS preset (#27)

[dogmar]

Summary

• Adds a Panda CSS preset at @klinking/squircle/panda-preset covering the full 15-utility radius matrix, with property names mirroring Panda's border<Side>Radius / rounded<Side> convention (e.g. squircleTopLeftRadius shorthand squircleTopLeft).
• Refactors the shared formula into squircleCssObj() in variants.ts so the Tailwind plugin, the static-CSS generator, and the new Panda preset all share one source of truth. Tailwind output is byte-identical (existing 163 snapshot tests unchanged).
• Closes part of Extend squircle utilities to work in stylex and panda-css #27 — the StyleX half is split out into a separate WIP branch (#TBD) because StyleX's babel plugin rejects every function-call and computed @-rule key inside stylex.create(), which makes the originally-planned factory API non-viable. Research notes are checked in on that branch.

What ships

Property name (camelCase)	Shorthand	Targets
squircleRadius	squircle	border-radius (all four corners)
squircleTopRadius	squircleTop	top corners
squircleRightRadius / Left / Bottom / Start / End	corresponding shorthands	per-side aggregates
squircleTopLeftRadius / TopRight / BottomRight / BottomLeft	corresponding shorthands	per-corner physical
squircleStartStartRadius / StartEnd / EndStart / EndEnd	corresponding shorthands	per-corner logical
squircleAmount	squircleAmt	--squircle-amt + corner-shape

Plus a _squircleSupported condition registered for @supports (corner-shape: superellipse(2)) overrides.

Consumer setup

// panda.config.ts
import { defineConfig } from "@pandacss/dev";
import squirclePreset from "@klinking/squircle/panda-preset";

export default defineConfig({
  presets: ["@pandacss/dev/presets", squirclePreset()],
  // ...
});

<div className={css({ squircle: "md", padding: "4" })} />
<div className={css({ squircleTopLeft: "lg", squircleAmt: 3 })} />

@pandacss/dev is added as an optional peer dependency.

Test plan

• vp run @klinking/squircle#test:panda — 11 unit tests (preset shape + transform output snapshots + amtVar/rVar option threading)
• vp run @klinking/squircle#test:plugin — existing 77 Tailwind tests unchanged
• vp run @klinking/squircle#test:css — existing 83 generated-CSS tests unchanged
• vp lint clean
• tsc --noEmit clean
• Manual smoke in a real Panda project (apply the preset, confirm squircle: "md" produces border-radius: var(--radii-md) + @supports-gated correction in DevTools)

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