fix(core): clip transparent borders to scissor

[opencode-agent]

Summary

• clip drawBox geometry to the active scissor before any border rendering path runs
• prevent the transparent-border fast path from writing raw cells outside a scroll viewport
• add both a native buffer invariant test and a high-level ScrollBoxRenderable regression that places an oversized transparent bordered child above a later sibling

Cause

drawBox only checked whether its bounds overlapped the current scissor. Its transparent-border fast path then wrote directly into the buffer, bypassing the point-level scissor validation used by normal cell writes. A tall bordered child could therefore paint through its scroll viewport and into later UI such as a composer.

Verification

• high-level regression fails on origin/main and passes with this change
• bun test src/tests/scrollbox.test.ts — 47 passed
• bun run test:native — 1,877 passed, 5 skipped
• bun run fmt:check
• bun run lint
• full repository build
• verified against the original OpenCode long-message + scroll-to-top reproduction

Requested by: @kommander (Sebastian via Slack)

[simonklee]

#1352 already fixes the same border clipping issue. This change can also move
box titles when a box is clipped. Please continue this work in #1352.