fix(client): retry transient content-chunk PUTs on wasm + native (0.6.10)

[ehsan6sha]

Problem

Large (chunked) uploads could fail on a single sporadic chunk-PUT drop (ERR_CONNECTION_CLOSED / transient 5xx under the 16-wide concurrent burst). The content-chunk PUT path (put_object_chunked_internal → put_object_with_metadata → request()) had no retry on either target: the blob-backend retry only wraps __fula_forest_v7_nodes/, and retry_idempotent only wrapped the S3-multipart path and collapsed to a single attempt on wasm for lack of a sleep primitive. Small (1-chunk) files almost never hit a drop; large ones did — worst on the web (FxFiles #50).

Fix

• multipart::retry_idempotent: unify the loop across targets; add a wasm backoff sleep via gloo-timers (TimeoutFuture); make it pub(crate).
• is_transient: un-gate for wasm (its body is already wasm-aware) so the unified retry can classify there.
• put_object_chunked_internal: wrap each chunk PUT in retry_idempotent (4 attempts, exp backoff capped 5s). Safe — chunk keys are content-addressed (idempotent); per-attempt clones are cheap (Arc / Bytes / small structs).
• add gloo-timers (wasm32 target); bump workspace version 0.6.9 → 0.6.10.

Tests / verification

• chunk_put_retries_transient (new): wiremock injects one transient chunk 503; asserts the chunked upload survives via retry. RED before the fix, GREEN after (verified).
• chunk_retry_real_server_e2e (new, #[ignore]): 3 MiB chunked upload + byte-identical round-trip against the live master with Mode A creds — passed.
• Full fula-client suite green (208 lib + 4 blob-backend retry + 1 chunk-retry, 0 failed).
• cargo check --target wasm32-unknown-unknown -p fula-client clean.

What's proven vs inferred (honest)

The native unit test proves the retry logic on native. The wasm side is compile-verified (cargo check for wasm32) — fula-api has no #[wasm_bindgen_test] infra, so wasm runtime retry is inferred from the now-shared loop (same code path, only the sleep differs by target). The real-server E2E proves the upload path is intact end-to-end on native.

Scope note

The per-file index-object PUT (written after the chunks) is the same idempotent, currently-unretried pattern. Left out of scope here (the reported failure + the test target the content chunks) — a one-line follow-up can wrap it in the same retry_idempotent.

Release

Closes the SDK side of FxFiles #50. Going live on the FxFiles web needs the 0.6.10 publish + flutter-wasm-pkg.zip release asset, after which FxFiles bumps fula_client + runs tools/sync-wasm-pkg.ps1.

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