Problem: Combined diff test fixtures used `ghi9012` as a result hash,
but `g`, `h`, `i` are not hex digits (`%x` matches `[0-9a-fA-F]`).
The `%x+` pattern in `highlight.lua` correctly rejected this, so the
manual `@constant.diff` extmark was never set. The existing assertion
passed anyway because the diff grammar's captures on parent hashes
(`abc1234`, `def5678`) satisfied the row-level check.
Solution: Replace `ghi9012` with valid hex `a6b9012` in all fixtures.
Tighten the result hash assertion to verify exact column range (23-30)
so it cannot be satisfied by parent hash captures.
Barrett Ruth
83c17aca67
