refactor(highlight): unified per-line extmark builder (#144)

## Problem

`highlight_hunk` applied DiffsClear extmarks across 5 scattered sites
with
ad-hoc column arithmetic. This fragmentation produced the 1-column
DiffsClear
gap on email-quoted body context lines (#142 issue 1). A redundant
`highlight_hunk_vim_syntax` function duplicated the inline vim syntax
path,
and the deferred pass in init.lua double-called it, creating duplicate
scratch
buffers and extmarks.

## Solution

Reorganize `highlight_hunk` into two clean phases:

- **Phase 1** — multi-line syntax computation (treesitter, vim syntax,
diff
grammar, header context text). Sets syntax extmarks only, no DiffsClear.
- **Phase 2** — per-line chrome (DiffsClear, backgrounds, gutter,
overlays,
  intra-line). All non-syntax extmarks consolidated in one pass.

Hoist `new_code` to function scope (needed by `highlight_text` outside
the
`use_ts` block). Hoist `at_raw_line` so Phase 1d and Phase 2b share one
`nvim_buf_get_lines` call.

Delete `highlight_hunk_vim_syntax` (redundant with inline path). Remove
the
double-call from the deferred pass in init.lua.

Extend body prefix DiffsClear `end_col` from `qw` to `pw + qw`, fixing
the
1-column gap where native treesitter background bled through on context
lines
in email-quoted diffs (#142 issue 1).

### Email-quoted diff support

The parser now strips `> ` (and `>> `, etc.) email quote prefixes before
pattern matching, enabling syntax highlighting for diffs embedded in
email
replies and `git-send-email` / sourcehut-style patch review threads.
Each hunk stores `quote_width` so the highlight pipeline can apply
`DiffsClear` at the correct column offsets to suppress native treesitter
on quoted regions.

Closes #141

### #142 status after this PR

| Sub-issue | Status |
|-----------|--------|
| 1. Col gap on context lines | Fixed |
| 2. Bare `>` context lines | Improved, edge case remains |
| 3. Diff prefix marker fg | Not addressed (follow-up) |

flake.nix

@@ -13,24 +13,41 @@
       ...
     }:
     let
-      forEachSystem = f: nixpkgs.lib.genAttrs (import systems) (system: f nixpkgs.legacyPackages.${system});
+      forEachSystem =
+        f: nixpkgs.lib.genAttrs (import systems) (system: f nixpkgs.legacyPackages.${system});
     in
     {
+      formatter = forEachSystem (pkgs: pkgs.nixfmt-tree);
+
       devShells = forEachSystem (pkgs: {
-        default = pkgs.mkShell {
-          packages = [
-            (pkgs.luajit.withPackages (
+        default =
+          let
+            ts-plugin = pkgs.vimPlugins.nvim-treesitter.withPlugins (p: [ p.diff ]);
+            diff-grammar = pkgs.vimPlugins.nvim-treesitter-parsers.diff;
+            luaEnv = pkgs.luajit.withPackages (
               ps: with ps; [
                 busted
                 nlua
               ]
-            ))
-            pkgs.prettier
-            pkgs.stylua
-            pkgs.selene
-            pkgs.lua-language-server
-          ];
-        };
+            );
+            busted-with-grammar = pkgs.writeShellScriptBin "busted" ''
+              nvim_bin=$(which nvim)
+              tmpdir=$(mktemp -d)
+              trap 'rm -rf "$tmpdir"' EXIT
+              printf '#!/bin/sh\nexec "%s" --cmd "set rtp+=${ts-plugin}/runtime" --cmd "set rtp+=${diff-grammar}" "$@"\n' "$nvim_bin" > "$tmpdir/nvim"
+              chmod +x "$tmpdir/nvim"
+              PATH="$tmpdir:$PATH" exec ${luaEnv}/bin/busted "$@"
+            '';
+          in
+          pkgs.mkShell {
+            packages = [
+              busted-with-grammar
+              pkgs.prettier
+              pkgs.stylua
+              pkgs.selene
+              pkgs.lua-language-server
+            ];
+          };
       });
     };
 }