feat(highlight): add treesitter context padding from disk

Problem: treesitter parses each diff hunk in isolation, so incomplete
syntax constructs at hunk boundaries (e.g., a function definition with
no body) produce ERROR nodes and drop captures.

Solution: read N lines from the on-disk file before/after each hunk and
prepend/append them as unmapped padding lines. The line_map guard in
highlight_treesitter skips extmarks for unmapped lines, so padding
provides syntax context without visual output. Controlled by
highlights.context (default 25, 0 to disable). Also applies to the vim
syntax fallback path via a leading_offset filter.

README.md

@@ -41,12 +41,10 @@ luarocks install diffs.nvim
 
 ## Known Limitations
 
-- **Incomplete syntax context**: Treesitter parses each diff hunk in isolation
-  without surrounding code context. When a hunk shows lines added to an existing
-  block (e.g., adding a plugin inside `return { ... }`), the parser doesn't see
-  the `return` statement and may produce incorrect highlighting. This is
-  inherent to parsing code fragments—no diff tooling solves this without
-  significant complexity.
+- **Incomplete syntax context**: Treesitter parses each diff hunk in isolation.
+  To improve accuracy, `diffs.nvim` reads lines from disk before and after each
+  hunk for parsing context (controlled by `highlights.context`, default 25).
+  This resolves most boundary issues. Set `highlights.context = 0` to disable.
 
 - **Syntax flashing**: `diffs.nvim` hooks into the `FileType fugitive` event
   triggered by `vim-fugitive`, at which point the buffer is preliminarily