feat: trace batch writes

[mag1cfrog]

Summary

• add stable batch write trace names and phase names for batch schema validation, value conversion, direct encoding, and packet write failures
• emit sanitized outer batch write telemetry for baseline, direct framed, and direct raw paths
• record attempted batch ordinal, batch shape, empty flag, pre-failure stats, successful cumulative WriteStats, result status, diagnostic codes, and elapsed time
• cover success, empty batches, validation failures, conversion failures, send failures, direct backends, and row-value redaction with unit fakes

Notes

This PR instruments the outer write_batch boundary. Direct raw per-packet and per-encoded-chunk telemetry remains scoped to #146.

Closes #145.

Validation

• cargo fmt --all --check
• cargo check --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• git diff --check