refactor: rename 'period' to 'segment' throughout codebase
Replaces all uses of "period" (referring to HHMMSS_LEN timestamped
duration folders) with "segment" for better clarity. The term "segment"
more accurately describes these discrete time-bounded recording windows.
Changes:
- Rename core functions: period_key() → segment_key(), period_parse() → segment_parse()
- Update all CLI tools: --period → --segment flag
- Update Callosum event protocol: period field → segment field
- Update documentation: JOURNAL.md and CALLOSUM.md formal definitions
- Rename all internal variables, methods, and test fixtures
- Preserve unrelated uses: "periodic" (frequency), "trailing period" (punctuation)
All 438 tests passing. No functional changes.