fix(find): fall back to raw find for unsupported flags

[mariuszs]

Problem

rtk find errors out when given native find features it can't model in its structured walker — compound predicates (-not, -or, -and), actions (-exec, -execdir, -delete, -print0), and time/size/perm tests (-size, -mtime, -perm, …):

$ rtk find . -name '*.tmp' -exec rm {} \;
Error: rtk find does not support compound predicates or actions (e.g. -not, -exec). Use `find` directly.

This forces users to reach for rtk proxy find ... | rtk proxy xargs ... to get any result, and it contradicts RTK's fallback principle: if a filter can't handle the input, run the real command unchanged rather than blocking the user.

Fix

run_from_args now detects unsupported flags up front and falls back to the real find via core::runner::run_passthrough:

• streams output unchanged (inherits stdin),
• tracks the invocation at 0% savings (visible in rtk gain --history, like rtk proxy),
• returns the child exit code, propagated through the single std::process::exit in main() like every other command (run_from_args now returns Result<i32>).

Supported syntax (-name, -iname, -type, -maxdepth, RTK shorthand) is unaffected — it still goes through the compact filter.

Tests

• unsupported_flags_detected / supported_flags_not_flagged_as_unsupported — flag classification
• run_from_args_falls_back_for_unsupported_flags — -not now runs the real find and returns Ok(0) instead of erroring
• Removed the two parse_*_rejects_* tests that asserted the old blocking behavior

Gate (cargo fmt --all --check && cargo clippy --all-targets && cargo test --all) is green on develop: 1983 passed, 7 ignored, 0 clippy warnings.