perf: increase InitialContractTransferByteCount to 256KiB

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Increases the initial contract size to reduce negotiation overhead. At the current 16KiB limit, the 80% ContractFillFraction threshold (~13KB) is often too small to accommodate a final ~1.4KB packet, leading to frequent contract exhaustion logs and performance bottlenecks.

This issue was originally investigated and reported by UnicoreX (Kevin) in the community Discord. Their analysis correctly identified the InitialContractTransferByteCount bottleneck for provider throughput.

Link to original bug report: https://discord.com/channels/807713145006850048/1412465917765353472/1496715109198729216

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@xcolwell

Updated PR#177, PR#180, and PR#182 to resolve merge conflicts and ensure they're all mergeable:

#177 Rebased onto current origin/main.
The single-line change (InitialContractTransferByteCount: kib(16) → kib(256)) is now cleanly applied to the refactored
DefaultContractManagerSettingsWithBufferSize() function.

#180 Merged origin/main to resolve conflicts in transfer_contract_manager.go.
All changes from both the log-spam-fix branch and upstream are preserved (OOB backoff + linger + queue timeout + control sync).

#182 No action needed (already mergeable).

All three are now green and ready for review. Thanks!