Fix Chisel session ID collisions and use path-safe joins

[viktorking7]

• Reworks next_cached_session() to compute the next ID as max(existing)+1 instead of entry count.
• Replaces string concatenation with Path::join in write() and load() for cross-platform path safety.
• Prevents accidental overwrites when gaps exist in chisel-<n>.json numbering.

[viktorking7]

@DaniPopes review it please

[DaniPopes]

no