Source: Implement line/column lookup from Span

[pdaxt]

Component

Foundation / Source

Task

Implement line/column lookup from byte offset.

Acceptance Criteria

• line_col(offset: u32) -> (u32, u32) returns 1-indexed line and column
• line_start(line: u32) -> u32 returns byte offset of line start
• line_content(line: u32) -> &str returns the line content
• Cache line offsets for O(log n) lookups

Verify

let src = Source::new("test", "line1\nline2\nline3");
assert_eq!(src.line_col(6), (2, 1)); // start of line2
assert_eq!(src.line_content(2), "line2");

Security Testing (REQUIRED)

• Test with various line endings (\n, \r\n, \r)
• Test empty lines and trailing newlines
• Test offset at exact line boundaries
• Test out-of-bounds offsets
• Test with single-line files (no newlines)
• Verify binary search correctness

Ref: FOUNDATION.md