fix: support Rust char formatting

[swananan]

Summary

• recognize four-byte DW_ATE_UTF values as Rust char data throughout classification and capture
• format valid scalars with Rust-style quoting and escapes, with explicit markers for invalid data
• add runtime e2e coverage for ASCII, Unicode, escaped chars, and char arrays, plus user documentation

Validation

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test -p ghostscope-protocol --all-features --lib (87 passed)
• cargo test -p ghostscope-compiler --all-features --lib (171 passed)
• runner e2e test_rust_script_print_char_values (job e542742c4d4e, passed)

The full standard host e2e run encountered the existing test_cross_type_comparisons_local failure. The same failure was reproduced with an isolated rerun on this branch and on a clean latest-main checkout (baseline job e9e9ea6134f1), so it is unrelated to this change.