feat: add opt-in ASCII varchar string policy

[mag1cfrog]

Summary

• Add an opt-in StringPolicy::AsciiVarChar(n) mapping for Arrow string columns.
• Emit bounded SQL Server varchar(n) schemas and single-byte TDS payloads.
• Reject invalid widths, non-ASCII values, and oversized values before sending any rows.
• Support baseline token-row, direct framed bulk, and direct raw bulk writers.
• Add documentation, diagnostics, observability, and SQL Server integration coverage.

Why

The existing Unicode-safe default maps strings to nvarchar(max). Applications whose string data is guaranteed to be ASCII can use a bounded varchar(n) representation to reduce wire and database costs without silently losing data. The new policy is explicit so existing callers keep their current Unicode behavior.

Validation

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo xtask sqlserver-test