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.
