saf 2.0.0: single-class SAF API - #44

Reformat 11 v2 lib/test files with dart format so the whole-repo
dart format --set-exit-if-changed gate passes under Dart 3.11.5.
Formatting-only changes (line wrapping, trailing commas); no logic
or public API changes.

… event-channel handler leak

Found by pre-merge adversarial audit:
- copyOrMove: accumulate bytes across files so directory-copy progress is a
  running total instead of resetting to 0 per file
- pickFile/pickFiles: persist READ only (was READ|WRITE, which threw on
  read-only providers and silently persisted nothing)
- SessionManager: unregister the per-session EventChannel handler on cancel

…d footer

…rtdoc API under /api

saf-v2 already contains all of master's v1.0.6 content plus the 2.0.0 work;
recorded as an ours-merge to resolve the squash-history divergence so the PR
is conflict-free. No file content changes.

….saf

De-brands the abandoned ivehement domain. Internal Kotlin package + Gradle
namespace + AndroidManifest + pubspec plugin.package only; the Dart import
stays package:saf and the method-channel strings are unchanged, so runtime
behavior is identical (APK builds; full device test already passed).

…rite

Resolves the two deferred audit findings:
- readFileStream is now pull-based (openReadSession/readSessionChunk/
  closeReadSession) so native never reads ahead of the Dart consumer — no
  unbounded buffering / OOM on very large files.
- writeFileBytes/startWriteStream/pasteLocalFile best-effort truncate the
  output on overwrite so shorter content leaves no stale trailing bytes on
  providers that ignore the 'wt' flag.

Device-verified on Motorola Edge 40 (Android 15): pull read returns correct
chunks; overwrite produces correct file sizes.

…atsApp/hidden-folder recipe

…o the app dir

Restores the classic legacy cache() workflow (e.g. pulling WhatsApp .Statuses
out to save/share) on the v2 primitives (list + copyToLocalFile). Adds an
example 'Copy->local' button + docs. Device-verified on Motorola Edge 40:
copies land in the app external dir.