Add managed scalar callback foundation

[awakecoding]

Summary

• Port the managed scalar callback foundation from the all-in-one advanced APIs work.
• Add scalar callback ABI/value types, registration/unregistration, scalar runtime invocation, result conversion, deterministic metadata, and prepared-statement invalidation.
• Add focused scalar callback integration coverage for fixed and variadic callbacks, argument conversion, null/text/blob/error result conversion, callback error propagation, deterministic metadata, unregister/re-register invalidation, and context/value destructor counts.

Scope

This PR is limited to managed scalar callback support. It intentionally does not include aggregate support, custom collations, extension-loading changes, SDK-kit changes, .NET exposure/facade/raw changes, workflow changes, or simulator changes.

Validation

• cargo fmt --check
• git diff --check
• cargo check -q -p core_tester --features pure-rust-crypto --test integration_tests
• cargo clippy -q -p core_tester --features pure-rust-crypto --test integration_tests -- --deny=warnings
• cargo test -q -p core_tester --features pure-rust-crypto --test integration_tests managed_scalar -- --nocapture

An initial targeted test attempt failed during compilation with rustc-LLVM ERROR: IO failure on output stream: no space on device. That was environmental: build output was moved off the nearly-full C: drive, and all validation above passed afterward.

[awakecoding]

Closing in favor of an upstream PR from the unprefixed branch split/core-managed-scalar-callbacks.