fix(highlight,parser,init): line_hl_group, clear_end, did_filetype, email-quoted diffs

Problem: line backgrounds used `hl_group + end_row` multirow extmarks
vulnerable to adjacent `clear_namespace`. `clear_end` was off by one.
`vim.filetype.match` returned nil for function-handled extensions
(`.sh`, `.bash`, etc.) when `did_filetype() != 0`. Parser didn't
handle email-quoted diff prefixes (`> `).

Solution: use `line_hl_group` single-point extmarks. Fix `clear_end`
to `hunk.start_line + #hunk.lines`. Override `vim.fn.did_filetype`
via `rawset` during retry. Strip `> ` quote prefixes in parser and
store `quote_width` per hunk.

README.md

@@ -17,6 +17,7 @@ with language-aware syntax highlighting.
 - `:Gdiff` unified diff against any revision
 - Background-only diff colors for `&diff` buffers
 - Inline merge conflict detection, highlighting, and resolution
+- Email-quoted diff highlighting (`> diff ...` prefixes, arbitrary nesting depth)
 - Vim syntax fallback, configurable blend/priorities
 
 ## Requirements