🐛 Nested clip regions leak (clip is a single rect, not a stack)

[natemoo-re]

• Package: clayterm
• Version: v0.6.0 (reproduced on main @ d75fb4b)
• Runtime: Deno 2.x
• OS: platform-independent (WASM); reproduced on macOS

Describe the bug

We track a single active clip rect instead of a stack, so nesting one clip inside another corrupts clipping for later siblings. When an inner clip closes, clipping switches off entirely rather than restoring the parent rect, so anything drawn after leaks past the outer bounds. With an outer clip of height 4 (rows 0-3) holding an inner clip plus a tall sibling, the sibling's ZZZZ/WWWW render at rows 4-5 (both should be blank). Horizontally it leaks the same way: an outer width-4 clip with a narrower inner one renders a later sibling as all 8 columns (SSSSSSSS) instead of SSSS. Single-level clipping already works.

To Reproduce

The vertical case leaks ZZZZ/WWWW onto rows 4-5; the horizontal case renders the sibling as SSSSSSSS instead of SSSS .

Expected behavior

Clip regions should form a stack. Entering a clip pushes intersect(parent active rect, child box); leaving pops it, restoring the parent rect for subsequent siblings, so an inner clip never disables the outer one. The grid should be:

AAAA
BBBB
XXXX
YYYY

Additional Information

Failing test case on nm/repro/nested-clip (test · diff). Clay emits balanced SCISSOR_START/SCISSOR_END pairs in DFS order, so the render-command walk just needs to maintain a stack. The fix likely lives in src/clayterm.c:621 (SCISSOR_START overwrites the active rect; should push intersect(parent, child)) and src/clayterm.c:628 (SCISSOR_END sets clipping = 0; should pop and keep clipping on while the stack is non-empty). The scalar clip state at src/clayterm.c:48-50 needs a small fixed-depth rect stack (~16 is ample); the setcell test at src/clayterm.c:70-86 stays correct once sourced from the stack top. C-only, no wire/JS/public-API change; depth-1 nesting reduces to today's single-rect behavior.