fula-client 0.6.14: wasm/web cancellable chunked upload (mirror native cancel)

[ehsan6sha]

Summary

Adds cooperative cancel to the wasm/web (non-resumable) chunked-upload path, mirroring the SDK's existing native resumable cancel. This lets the web Sync Queue abort an in-progress large upload (Stage 2 of the web upload-cancel/resume plan; resumable is 0.6.15).

Changes

• put_object_chunked_internal + the shared put_object_flat_deferred_locked take an optional cancel: Option<Arc<AtomicBool>> (the 3 existing callers pass None — unchanged).
• The per-chunk buffer_unordered closure checks the flag before the chunk PUT: a not-yet-started chunk short-circuits with ClientError::Cancelled; chunks already in flight (≤16) finish. A second check before the index PUT covers cancel-after-all-chunks. The existing post-loop compensating-delete removes the uploaded (now-unreferenced) chunks — same cleanup as the failure path.
• New pub put_object_flat_with_progress_cancellable(... progress, cancel).
• FRB put_flat_with_progress_cancellable(&ProgressHandle, &CancelHandle) shares the CancelHandle's Arc<AtomicBool> with the upload.
• Workspace version → 0.6.14.

Validation

• Compiles clean: fula-client native + wasm32, fula-flutter native.
• Unit test: a pre-set cancel flag aborts the chunked upload with Cancelled.
• Regression: progress + native-resumable progress tests pass.
• Reviewed: built-in advisor (implementation-level concurrency analysis) — sound. External concurrency reviewers (Gemini + Qwen-coder) in flight; codex quota-limited until July.

NON-resumable

A cancelled upload restarts from scratch. wasm resumable (IndexedDB manifest, BAO integrity) is the next change (0.6.15).

Dart wiring (post-publish, not in this PR)

When wiring the web Sync Queue against 0.6.14:

1. Cancelled must bypass retryAsync/isTransientUploadError and not be re-enqueued by _pump (else a cancelled upload re-runs).
2. Cancelled must surface as a distinguishable "cancelled", not a generic failure toast.
3. The real gate is a live mid-flight web E2E (cancel partway → stops + cleans orphaned chunks + queue advances).

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