## Problem
Two-pass rendering (Pass 1: backgrounds + intra-line; Pass 2:
treesitter) caused Pass 2 to re-apply all extmarks that Pass 1 already
set, doubling the extmark count on affected lines.
## Solution
Add `syntax_only` mode to `highlight_hunk`. When `syntax_only = true`,
only treesitter syntax and content `DiffsClear` extmarks are applied —
backgrounds, intra-line, prefix clears, and per-char prefix highlights
are skipped. Pass 2 now uses `syntax_only = true` and no longer calls
`nvim_buf_clear_namespace`, so Pass 1's extmarks persist while Pass 2
layers syntax on top.
Closes#143
90b312e8df