feat: FFI MVP and incremental classification hardening

[Hyper66666]

Summary

• land FFI MVP across lexer/parser/AST/HIR/typeck/codegen with tests and examples
• add FFI docs (docs/ffi.md) and example suite under examples/ffi/
• fix fast interface fingerprint to avoid body-only edits being misclassified as interface changes
• add regression coverage in tools/sgc/src/tests.rs

Validation

• cargo check -p sengoo-compiler --all-targets
• cargo check -p sgc
• cargo test -p sengoo-compiler --no-run
• cargo test -p sgc --no-run