transfer: SkipTx=true waits for log entry, doesn't fall back to re-submit (closes #150)

[0g-peterzhb]

Closes #150.

What

After #145 + #149, `Client.SplitableUpload`'s retry correctly sets `opt.SkipTx = true` when the previous attempt landed a Flow.submit. But `uploadSlow` / `uploadFast` gate the skip on `checkLogExistence` returning non-nil `info` — and on retry, the new `Uploader` is built against fresh storage nodes from the indexer that may not have synced the previous submit yet. `info == nil` → fresh `Flow.submit` despite `SkipTx=true` → operator wallet pays twice.

Fix

Split the conditional so `SkipTx=true` means "the caller asserts the entry is on chain; wait for storage nodes to see it; do NOT re-submit." When `info == nil` under `SkipTx=true`, `waitForLogEntry` instead of falling back to submit. Same change in both branches.

```go
if !opt.SkipTx {
// submit
} else if info == nil {
info, err = waitForLogEntry(ctx, tree.Root(), TransactionPacked, 0, false)
if err != nil {
return ..., "SkipTx=true but log entry never appeared on storage node"
}
}
```

Behavior change for callers

A caller passing `SkipTx=true` when the entry is NOT actually on chain used to fall back to a fresh submit; with this fix they would wait until ctx timeout. That's correct — `SkipTx` is a deliberate caller assertion that the entry exists. Silent re-submit was always a bug surface (operator wallet pays for a duplicate). The `0g-storage-s3` gateway, the main consumer of this SDK today, uses `SkipTx` exactly this way:

• Crash-resume from a recorded prior `flow_submit_attempts` row
• Retry inside `Client.SplitableUpload` after a previous attempt's submit succeeded (indexer: skip Flow.submit on retry after first submit succeeded #145)

Test plan

• `go build ./...` + `go vet ./...` clean against `origin/main`
• `go test ./transfer/...` green
• Re-run the s3-gateway's full-op + large-file testnet smokes — segment-upload retries should now land exactly one Flow.submit even if the new uploader's nodes haven't synced

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