Problem: `line_hl_group` bg unconditionally overrides `hl_group` bg
regardless of priority (neovim/neovim#31151). `DiffsAddText`/
`DiffsDeleteText` at p201 were invisible under `DiffsAdd`/`DiffsDelete`
`line_hl_group` at p200 because they operate on separate stacking layers.
Solution: replace `line_hl_group` with `hl_group` + `hl_eol` +
`end_col` for line backgrounds, putting them on the same layer as
intra-line highlights so priority governs stacking. Use single-row
extmarks (`end_row = buf_line`) to avoid the adjacent-hunk clearing
bug. Split `number_hl_group` into a separate extmark. Use raw
`DiffAdd.bg`/`DiffDelete.bg` for `DiffsAddText`/`DiffsDeleteText`
instead of alpha-blending toward Normal.bg, which produced
indistinguishable colors on dark themes.
## Problem
Email-quoted diffs (`> diff --git ...`, `> @@ ...`) from git-send-email
/ email reply workflows produce 0 hunks because the parser matches
patterns against raw lines containing `> ` quote prefixes. Closes#141.
## Solution
Strip the `> ` quote prefix before pattern matching in the parser. Store
`quote_width` on each hunk. In `highlight.lua`, offset all extmark
column positions by `qw` and expand `pw > 1` guards to `qw > 0 or pw >
1` for DiffsClear suppression. Clamp body prefix DiffsClear `end_col` to
the actual buffer line byte length for bare `>` lines (1-byte buffer
lines where `end_col = pw + qw` would exceed bounds and cause
`nvim_buf_set_extmark` to silently fail inside `pcall`).
15 new specs covering parser detection, stripping, false-positive
rejection, and highlight column offsets including the bare `>` clamp
edge case.
