Reject whitespace in relative interpreter paths

[jeet-dekivadia]

Summary

• reject relative interpreter tokens containing spaces or tabs so behavior does not depend on kernel shebang splitting rules
• keep the C, Rust, and reference Python implementations aligned on the short-term contract
• document the restriction and add shared regression coverage for both ambiguous whitespace characters

Fixes #4

Validation

• git diff --check
• rustfmt --edition 2018 --check rust/src/main.rs

Remote Linux validation

• git diff --check
• cargo fmt --manifest-path rust/Cargo.toml -- --check
• python3 test_relexec.py

[jeet-dekivadia]

Small follow-up while this is waiting for review. The patch is intentionally symmetric across the C parser, Rust parser, and Python reference harness: the only new rejection is a relative interpreter token containing a space or tab, because those tokens are ambiguous across shebang splitting paths.

Validation from preparation: git diff --check; cargo fmt --manifest-path rust/Cargo.toml -- --check; python3 test_relexec.py. The added shared regressions cover both ambiguous whitespace characters.