feat: per-file offset tracking for doc insertions

[lmmx]

Implements per-file offset tracking as described in #6.

Changes:

• Added file_offsets: HashMap<String, HashMap<i64, usize>> to AppState to track cumulative line additions per file
• rebuild_file_offsets() builds the map on each save, handling external module docstrings at line 0
• cumulative_offset() now queries the map filtered by file and line instead of summing globally
• External module docstrings correctly offset subsequent diagnostics in their target files

Example:

• src/foo.rs with 10-line module docstring from mod foo; declaration
• Adding docstring at line 20 uses offset +10
• Adding docstring at line 25 uses offset +11 (module + previous docstring)

Fixes #6