Removes the legacy JSON FFI boundary in favor of typed C/Go APIs.

Also fixes OCI layer extraction for container-root absolute symlinks so Alpine command execution can resolve /bin/sh while preserving host escape protections.

Validation:

• cargo fmt --check
• BOXLITE_DEPS_STUB=1 cargo clippy -p boxlite --tests -- -D warnings
• BOXLITE_DEPS_STUB=1 cargo clippy -p boxlite-ffi -p boxlite-c --tests -- -D warnings
• cargo build -p boxlite-c
• ctest --test-dir sdks/c/tests/build --verbose --output-on-failure -R test_execute_cmd
• ctest --test-dir sdks/c/tests/build --verbose --output-on-failure -R test_basic
• make lint:c
• go test -tags boxlite_dev -run 'Test(Error|Is|BoxOptions|RuntimeOptions|BuildCOptions|StateConstants|With)' .\n- go test ./cmd/setup