test sweep: cover src/editor/ tail paths (T-91)

Adds 6 tests in test/editor/registry_test.dart closing out the
EditorRegistry methods the existing suite didn't reach:

- activate(id): unknown id is a no-op (no event emitted); known id
  flips the active buffer and emits editor.active-changed.
- setContent(id, content, selection?): explicit selection clamped to
  new content length, missing selection falls back to clamping the
  existing one, missing id is a silent no-op. Each emits the
  editor.edited replace event.
- contentFromArgs(args): the content_b64 fallback when text is
  absent, plus the empty-args default.
- Selection.hashCode + .toString.

Coverage: src/editor/buffer.dart 20/24 -> 24/24; src/editor/registry
.dart 87/105 -> 105/105. Both at 100%.

Total coverage 74.79% -> 75.11%; floor bumped to 75.

Co-Authored-By: Claude Opus 4.7 (1M context) <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: 74
+coverage_floor: 75
 
 # Project metadata (was project.yaml, folded in per D-056).
 # version: above is the single source of truth. The Makefile reads