feat: MarkerInfo OR-Set facade + deleteDocument + delete-event visibility

[kitplummer]

Summary

Closes #2 (MarkerInfo OR-Set) and #3 (Delete operation).

• Facade: putMarker(MarkerInfo), markers getter, and generic deleteDocument(collection, docId) on PeatFlutterNode; MarkerInfo added to public API exports
• Example app: Markers card in Operations tab — drop pin (random DC-area coords), swipe-to-soft-delete (tombstone with deleted: true), live refresh on change events
• Activity tab: Hard-delete events (ChangeType.delete with raw == null) now surface with a (deleted) preview and red pill, instead of being silently discarded
• Tests: marker_info_test.dart (12 tests — JSON round-trip, copyWith, equality, soft-delete filtering) and delete_event_test.dart (5 tests — ChangeType codec, delete vs upsert distinction, remote delete origin)

Test plan

• flutter test — all 23 tests pass (12 marker + 5 delete + 6 existing)
• dart analyze — no errors (info-only pre-existing deprecation warnings)
• CI native build (arm64) passes
• Manual: start node, drop markers via + button, verify they appear in list, swipe to delete, confirm tombstone propagates (marker disappears from live list, change shows in Activity tab)

[peat-bot]

Peat QA Review (SHA: a2073da)

No findings.

Scope check — pure Dart facade + example wiring; no changes to native surface, FFI encode/decode paths, rust/Cargo.toml (still peat-ffi = "=0.2.10"), or platform build scripts. MarkerInfo, putMarker, getMarkers, and deleteDocument were already present in the generated lib/src/generated/peat_ffi.dart (native symbols uniffi_ffibuffer_peat_ffi_fn_method_peatnode_{put_marker,get_markers,delete_document} all resolve), so bindingsContractVersion=30 remains valid — the facade methods added in lib/src/peat_node.dart are thin pass-throughs. Public API export (lib/peat_flutter.dart) correctly adds MarkerInfo.

Behavioral notes verified:

• Marker soft-delete is a putMarker with deleted:true — CRDT-safe tombstone as ADR-035 requires (peat-mesh does not yet propagate ChangeEvent::Removed).
• Hard-delete events (ChangeType.delete, raw==null) now render with (deleted) preview + red pill; upsert path unchanged and still content-hash-deduped.
• Test coverage: marker_info_test.dart (JSON round-trip, copyWith with nullable sentinel, equality, soft-delete filtering) and delete_event_test.dart (ChangeType codec + delete/upsert distinction + remote-origin preservation) are pure-Dart and require no native lib; native round-trip is validated by the existing native-linux-arm64 CI job and just check-bindings.
• No security primitive, protocol/schema, cross-platform build, or FFI memory-safety surface touched.