#31754 test(chat): account for transcript grid gaps in scroll oracle

The upward scroll browser oracle treated every adjacent transcript row as flush, so the settled 24/32px grid spacing from #27580 looked like blank bands. This updates the test oracle to derive expected transcript gaps from rendered row kinds and computed grid tokens, including footer rows, and subtracts an expected settled gap at viewport boundaries. Missing rows, extra blank bands, viewport-edge blanks, anchor drift, fold continuity, and subpixel overlaps remain failures. Falsifying browser fixtures cover an accepted intentional gap, a missing row, and an oversized gap.

Validation at head 87fca5185dcde2612672884a0f242b23aab3453d:

• Focused browser suite: run 41386505809, 7 passed / 0 failed.
• bun run check: repository checks passed except unavailable vp commands in this lane (vp was not installed); no changed-file check failure.

agent-identity footer --repo manaflow-ai/cmux
Agent session: agent session