feat(core): SVG-based border rendering (v1.2.0) (#3)

* chore: update gitignore for CLAUDE.md patterns

- Add **/CLAUDE.md to ignore CLAUDE.md in all subdirectories
- Add CLAUDE.md.backup to ignore backup files

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Co-Authored-By: Claude <noreply@anthropic.com>

* feat(core): add SVG-based border rendering (Feature 006)

Add comprehensive border support for squircle elements:

- Solid, dashed, and dotted border styles
- Gradient borders with configurable color stops
- Anti-aliasing fix for dark backgrounds (SC-001)
- Data attribute support (data-squircle-border-*)
- Background/box-shadow capture and restoration
- Dynamic border updates via ck.update()
- Border width clamping (1-8px range)

New files:
- border.ts: SVG border renderer with clip-path approach
- border.test.ts: Comprehensive unit tests (50 tests)
- border-visual.test.ts: Playwright visual regression tests

API additions:
- SquircleConfig.border: { width, color, style, gradient, dashArray }
- BorderConfig, GradientStop types exported

Bumped version to 1.2.0

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* chore(ci): update bundle size target to 6KB for border support

Feature 006 adds ~0.8KB for SVG border rendering. Updates success
criteria from SC-002 (5KB) to SC-004 (6KB with border support).

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* fix(ci): make PR comment step non-blocking

PR comments require write permissions which may not be available
on fork PRs. The bundle size verification is the critical step.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* docs: update changelog and security audit for v1.2.0

- Add v1.2.0 section to packages/core/CHANGELOG.md with SVG border features
- Update root CHANGELOG.md with v1.2.0 link
- Update SECURITY-AUDIT.md date and version to v1.2.0

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Co-authored-by: Claude <noreply@anthropic.com>

Release @cornerkit/vue v1.0.0 - Vue 3 Integration

Release @cornerkit/svelte v1.0.0 - Svelte Integration

Release @cornerkit/react v1.0.0 - React Integration

Release @cornerkit/core v1.1.0 - Border support

Release @cornerkit/core v1.0.2 - Safari clip-path fix

Release @cornerkit/core v1.0.0 - Initial release