## 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 for the result hash 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 (cols 23-30) so it
cannot be satisfied by parent hash captures.
Closes#168
823743192a