feat: trace schema planning

[mag1cfrog]

Summary

Closes #143.

Adds stable structured tracing around Arrow-to-SQL Server schema planning without changing planning semantics or public API behavior.

Changes

• Add stable schema planning phase, span, and event marker names.
• Emit schema planning start, successful completion, and failure events.
• Record field count, mapping count, diagnostic counts, profile fields, policy fields, and elapsed time.
• Record diagnostic codes and diagnostic field names for failure summaries.
• Record low-cardinality Arrow and MSSQL type family summaries on successful planning.
• Keep sensitive schema metadata and timezone strings out of tracing fields.

Safety

• Does not emit row values.
• Does not emit schema metadata values.
• Does not emit full Arrow DataType debug strings.
• Does not emit arbitrary SQL text, connection strings, passwords, or tokens.
• Does not install a global subscriber.

Validation

• cargo fmt --all --check
• cargo test -p arrow-tiberius schema::table_mapping
• cargo check --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• git diff --check main...HEAD