Provides two boundary-ID utilities used across all write paths:
new_boundary_id() — 8-hex-char UUID v4 prefix (length controlled by BOUNDARY_ID_LEN).
new_boundary_id_with_summary(summary) — same ID optionally suffixed with a 3-word,
20-char-max slug derived from summary (format: a0cfeb34:boundary-fix).
format_boundary_marker(id) — renders the canonical HTML comment form
<!-- agent:boundary:<id> --> used in document component boundaries.
BOUNDARY_ID_LEN = 8 is a public constant; callers may read but should not override.