Add append loads to the sqlserver connector

[victorchutw]

Implements ADR-0064 (append) with ADR-0065's append rules live. Part of the SQL Server destination cycle (map #106, sliced from #113).

Blocked by #136

Context

Append mirrors ADR-0047 verbatim on SQL Server: independent per-chunk bulk loads, auto-committed, committed prefix persists on failure. The table must already exist — append never bootstraps.

The contract

• begin(append): the table must pre-exist; an absent table fails destination_write_failed with before-append wording and the existing before-session posture (the DuckDB append/merge precedent). Present → introspect + Accept Family with append's rules (Validate existing SQL Server tables against the Accept Family #135): mapped IDENTITY rejected, mapped DEFAULT-carrying column with a nullable field rejected, extra columns per the extra-column rule. No transaction is opened.
• write_chunk: one auto-committed bulk_insert per chunk (one INSERT BULK batch). The spike proved a failed batch is atomic — zero rows — so a failed load keeps exactly its committed prefix.
• commit: finalize only; write facts atomicity: "best_effort", strategy: "bulk_insert" (new string, additive).
• Extra columns: the stream carries NULL for them — which the bulk path documentedly replaces with the column's DEFAULT — or a typed placeholder the server discards for IDENTITY (Validate existing SQL Server tables against the Accept Family #135's rules make this reachable only where faithful).
• Modes: supported_load_modes += append. Classification stays Terminal in this slice (the transience slice flips append's decided sites).

Acceptance criteria

1. Gated live tests:
   • Two loads append cumulatively (counts add up; no delete/replace).
   • Append onto a missing table fails before any write with the before-append posture.
   • Committed prefix: with a small configured chunk_rows, a load whose later chunk carries a >32,767-character value fails, and exactly the earlier chunks' rows are present — report says best_effort, failure surfaced, earlier rows queryable.
   • Extra destination columns: nullable, DEFAULT-carrying, and IDENTITY extras accept the load and take NULL/default/generated values respectively.
   • Rejections: a mapped-IDENTITY table and a mapped DEFAULT+nullable-field table each fail incompatible_destination_table at begin.
2. Report facts and exit codes as specified; serverless cargo test --locked green with the new tests ignored.
3. CHANGELOG.md Unreleased entry: append lands on sqlserver.