fix(core): align bordered hit-grid clipping (#1441)

## Summary
- align hit-grid scissor coordinates with bordered content bounds
- preserve correct clipping for buffered, nested, partial-border, and
scrollable containers
- add regression coverage for final-row and final-column hit targets

Fixes anomalyco/opentui#1231

## Verification
`bun test src/tests/scrollbox-hitgrid.test.ts`

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Co-authored-by: Simon Klee <hello@simonklee.dk>