fix(rewrite): CycleInterval::len to safe pattern and correct bounds

[jrootcache]

Problem

The CycleInterval::len() method used invalid Rust syntax to conditionally extract start and end, potentially causing compilation issues and incorrect interval length computation.

Fix

Refactor CycleInterval::len() to a simple, idiomatic match over (start, end) and compute length only when both bounds exist and end >= start.

Why this is safe

• Minimal change confined to a single method with no API changes.
• Preserves existing behavior for valid intervals and gracefully returns 0 when bounds are missing or inverted.
• Avoids unstable or illegal syntax, ensuring compatibility with stable Rust compilers.