fix: use lookbehind in highlightSpans to fix adjacent spans and start-of-line

[dpecos]

Summary

• Replaces the consuming preceding-character group ([^\])with a negative lookbehind(?<!['`'\])inhighlightBlockSpans`
• Fixes both issue Bug: highlightSpans fails when backtick is the first character on a line #2 (span at start of line) and issue Bug: highlightSpans incorrectly captures trailing parenthesis #3 (trailing parenthesis captured) in a single, cleaner change
• Adds highlightSpans.test.ts with 8 tests covering both bugs and surrounding edge cases

Root cause (both bugs share the same origin)

The old pattern ([^\])`([^\`]+?)` consumed one character before the opening backtick as group 1 (e`). This caused:

Issue #2 – no preceding character at start of line → no match at all.

Issue #3 – with adjacent spans like `foo`(`bar`):

• The engine couldn't match the first span's opening backtick (nothing before it, or backtick before it)
• It found o (last char of foo) as e, then treated foo's closing backtick as the opening of a new span
• Content [^\]+?then matched(`, which was "highlighted" — the bug

Fix

// before — consumes preceding char, breaks at start-of-line and adjacent spans
pattern = /([^`])`([^`]+?)`/g;

// after — lookbehind: opening backtick must not be preceded by ` or \
pattern = /(?<![`\\])`([^`]+?)`/g;

The lookbehind also naturally handles the escape case: \` is no longer treated as an opening delimiter, so the separate if (e === '\\') return m.slice(1) branch is no longer needed.

Closes #2
Closes #3