Harden SQL insert hot path

Restore strict SQL insert scale coverage and reduce repeated INSERT overhead without relaxing row counts or timing budgets.

Cache parameterized INSERT plans with DDL invalidation, route PK and UNIQUE checks through one indexed constraint path, and avoid repeated hot-path schema and row clones.

Author: lucidprogrammer

crates/contextdb-core/tests/timestamp_audit.rs

@@ -117,15 +117,15 @@ fn timestamp_audit_no_new_txid_shaped_columns() {
         ("tests/acceptance/query_surface.rs".to_string(), 1114u32),
         (
             "tests/integration/indexed_scan_filter_tests.rs".to_string(),
-            1969u32,
+            1964u32,
         ),
         (
             "tests/integration/indexed_scan_filter_tests.rs".to_string(),
-            1995u32,
+            1990u32,
         ),
         (
             "tests/integration/indexed_scan_filter_tests.rs".to_string(),
-            2879u32,
+            2924u32,
         ),
         ("tests/integration/retention_tests.rs".to_string(), 1366u32),
         ("tests/integration/retention_tests.rs".to_string(), 1414u32),