fix: regex panic on hunk headers without line count #159

Bug Description

cora review panics with no group at index '4' when parsing diff hunk headers that omit line counts (e.g., @@ -1 +1 @@).

Evidence

thread 'main' (517534) panicked at regex-1.12.3/src/regex/string.rs:2024:32:
no group at index '4'
zsh: abort      RUST_BACKTRACE=1 cora review --base origin/develop

Trigger: cora review --base origin/develop

Root Cause

src/engine/diff_parser.rs lines 128-130 — direct indexing caps[N] on optional capture groups.

static RE_HUNK_HEADER: LazyLock<Regex> =
    LazyLock::new(|| Regex::new(r"^@@\s-(\d+)(?:,(\d+))?\s\+(\d+)(?:,(\d+))?\s@@\s?(.*)").unwrap());

// PANICS when optional group 2 or 4 didn't participate in match
let old_count: u32 = caps[2].parse().unwrap_or(1);  // line 128
let new_count: u32 = caps[4].parse().unwrap_or(1);  // line 130

Groups (?:,(\d+))? are optional. When git outputs @@ -1 +1 @@ (no ,1), groups 2 and 4 don't participate. Direct caps[N] indexing on non-participating groups panics in the regex crate.

Fix

Use .get(N) for optional groups:

let old_count: u32 = caps.get(2).and_then(|m| m.as_str().parse().ok()).unwrap_or(1);
let new_count: u32 = caps.get(4).and_then(|m| m.as_str().parse().ok()).unwrap_or(1);

Impact

• Blocks all PRs — Cora Review is a required CI check
• Affects any diff with single-line hunks (very common)