[beinan]

Refs #378

Summary

WAL framing can write the length, header and each shared payload with separate socket calls. Add an opt-in vectored-io helper that sends the same bytes using at most 64 borrowed IoSlice descriptors, without concatenating payloads. Short writes, scalar fallback, errors and cancellation preserve exact body-byte accounting; prepared exchanges can send bodies separately after a grant.

This is a transport building block for the ingestion work tracked in #378. It does not enable distributed ingestion in the production writer or claim a throughput improvement. Incomplete exchanges still require discarding the connection, and socket acceptance is not a durability ACK.

Testing

• Eight focused tests cover partial writes, fragmented payload ownership, empty segments, scalar fallback, interrupted/error/cancelled sends, counter overflow, invalid progress and real TCP framing.
• Enable the feature tests in the existing WAL CI step; all-feature coverage also includes them.
• Local repository check script passed: formatting, workspace Clippy/tests (596 passed; existing ignored suites excluded), Python tests (229 passed, 2 skipped, 1 expected failure), Ruff and Pyright. The HTTP tests used a server built from this worktree.
• cargo test -p lance-context-wal --features vectored-io --lib and feature-enabled Clippy with --all-targets -- -D warnings passed.