fix: preserve unread input in local streaming writes - #295

Local PutStream writes should consume only the bytes a caller has not already read. The local first attempt starts at the reader's call-entry position, and retries return to that same position. A positioned reader that relied on the old local rewind will now store only its unread suffix. For active calls whose starting position cannot be obtained, storage gets one attempt; a failed retry seek returns an error instead of writing a different slice. No public signature or wire format changes, and remote forwarding is unchanged. The embedded API comment and guide document the local behavior.

TestPutStreamReaderPosition covers positioned bytes.Reader and file inputs, zero and end positions, reader-only and pipe-backed readers, cancellation, and a Seeker whose position query blocks. TestPutLocalReaderRetryPosition injects a retryable failure after a partial read and checks replay. TestPutLocalReaderWithoutPositionAttemptsOnce and TestPutLocalReaderRetrySeekFailure cover unavailable positions and failed restoration. The focused server Put suite exercises the unchanged unary bytes-reader and io.PipeReader callers.

This condition holds with this change and does not hold at 0759cbea25127e1e69a2ac0bb5a25bc429d32ab8: With a live context and a local embedded.Client.PutStream call, a successful write stores exactly the bytes from the reader's call-entry position through EOF; retryable storage failures replay from that offset when it is available, while readers with no obtainable starting position receive one storage attempt from the current position. A failed retry seek returns an error without committing another slice, and an already-canceled seekable call returns a timeout before invoking the reader's Seek method, without consuming or storing the input..

A standalone executable also drives embedded.Client.PutStream through a retryable metadata-commit failure after the body is consumed. The executable and its fault-injection helper are not included in the PR. The shipped operations test covers the separate partial-read retry case. The focused embedded and server Put tests passed, including the race-filtered Put path. TestPut_MetadataCommitFailureReclaimsRawFile passed; it verifies that a raw file is reclaimed after a failed metadata commit. The server build, lint, and license check also passed.

Caller incidence and storage or copying cost remain unmeasured.

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