feat(highlight): wire highlights.context config into treesitter pipeline

Problem: `highlights.context.enabled` and `highlights.context.lines` were
defined, validated, and range-checked but never read during highlighting.
Hunks inside incomplete constructs (e.g., a table literal or function body
whose opening is beyond the hunk's own context lines) parsed incorrectly.

Solution: `compute_hunk_context` reads the working tree file using the
hunk's `@@ +start,count @@` line numbers to collect surrounding code.
Files are read once and cached across hunks. `highlight_treesitter` accepts
an optional context parameter that prepends/appends context lines to the
parse string, offsetting capture rows so extmarks only land on hunk lines.

lua/diffs/parser.lua

@@ -15,6 +15,8 @@
 ---@field prefix_width integer
 ---@field quote_width integer
 ---@field repo_root string?
+---@field context_before string[]?
+---@field context_after string[]?
 
 local M = {}