test sweep: cover BufferRange family (T-91)

Adds test/terminal/buffer/range_test.dart — 38 unit tests covering
the small pure-Dart files in lib/src/terminal/src/core/buffer/:

- cell_offset.dart       23 / 23  (was 0 / 23)
- range.dart             13 / 13  (was 0 / 13)
- segment.dart           13 / 13  (was 0 / 13)
- range_line.dart        30 / 30  (was 0 / 30)
- range_block.dart       48 / 48  (was 0 / 48)

Total project line coverage 34.90% → 36.39%; coverage_floor in
pubspec.yaml bumped 34 → 36 in lockstep.

Tests exercise the abstract BufferRange operator==/hashCode/toString
via a local _StubRange (BufferRangeLine and BufferRangeBlock both
override those, so the base versions are otherwise unreachable —
worth a stub rather than carving the lines out of coverage). The
denormalized-input branches in Block contain/toSegments/extend get
explicit cases too.

Pure Dart, no Flutter dependency — uses package:test/test.dart and
runs in <100ms.

First batch under T-91; line.dart and buffer.dart land in subsequent
commits with their own floor bumps.

Co-Authored-By: Claude <noreply@anthropic.com>

pubspec.yaml

@@ -18,7 +18,7 @@ repository: https://github.com/postmeridiem/clide
 
 # Pre-push line-coverage floor. Ratchets up only — see D-66.
 # Reading: `awk -F: '/^coverage_floor:/ {gsub(/ /,"",$2); print $2}' pubspec.yaml`.
-coverage_floor: 34
+coverage_floor: 36
 
 # Project metadata (was project.yaml, folded in per D-056).
 # version: above is the single source of truth. The Makefile reads