feat(parser): resolve overlapping timesheet entries

Parallel work is logged as overlapping entries. resolve_overlaps()
splits the shared time equally using the midpoint of the overlap region:

- Partial overlap: the midpoint becomes the boundary between the two
  entries (earlier entry trimmed, later entry delayed).
- Full containment: the containing entry is split into two pieces
  surrounding the contained one, with the midpoint rule applied to
  the overlap region.

Open entries (no end time) are passed through unchanged.

resolve_overlaps() is called automatically in filter_rows_by_date,
filter_week_sections, and the --input single-day path in cli.py, so
all subcommands benefit without further changes.

src/timesheets/cli.py

@@ -26,6 +26,7 @@ from .parser import (
     filter_rows_by_date,
     filter_week_sections,
     parse_document,
+    resolve_overlaps,
 )
 from .projects import load_project_map
 from .utils import AmbiguousDateError, format_date, parse_date_arg
@@ -211,7 +212,7 @@ def _resolve_rows(
             except FileNotFoundError:
                 print(f"Error: file not found: {args.input}", file=sys.stderr)
                 sys.exit(1)
-        return parse_document(content.splitlines())
+        return resolve_overlaps(parse_document(content.splitlines()))
 
 
 def _resolve_week_sections(