Alternative: RepeatingWithin cycle kind (I notation) — subclasses Within

[rymiwe]

Summary

Alternative to #71 — same behavior, different class structure. This PR subclasses Within to make the relationship explicit, per jdowd's feedback on #69.

V1I24MF2026-03-31 = complete 1 every 24 months from March 31, 2026. After satisfaction, the consuming app calls reactivated_notation(completion_date) to start the next window.

Design: Subclass vs Sibling

	#71 (Interval < Cycle)	This PR (RepeatingWithin < Within)
Relationship to Within	Implicit (same implementation)	Explicit (inheritance)
Inherited from Within	Nothing	final_date, start_date, date_range
Overrides	N/A (standalone)	recurring?, to_s, extend_period, last_completed, expiration_of, satisfied_by?, start_date (nil-safe), final_date (nil-safe)
Liskov concern	None	recurring? returns opposite of parent
Developer signal	"This is its own thing"	"This is Within, but repeating"

Tradeoffs

Subclass (this PR):

• Makes Within relationship immediately apparent to developers
• Inherits final_date/start_date computation (DRY)
• But overrides 7 methods including core semantics (recurring?, satisfied_by?)
• Needs nil-safe overrides for start_date/final_date because Within assumes active state

Sibling (#71):

• Clean separation, no Liskov concern
• Self-contained — all behavior visible in one file
• But duplicates final_date/start_date one-liners from Within
• Relationship to Within only visible by reading both files

Bonus: reactivated_notation alias

Added reactivated_notation(date) as an alias for activated_notation(date). Same operation, but self-documenting at the call site:

# Activating a dormant cycle
notation = dormant_cycle.activated_notation(first_date)

# Re-anchoring a satisfied RepeatingWithin
notation = satisfied_cycle.reactivated_notation(completion_date)

Test plan

• All 209 gem specs pass (179 existing + 29 RepeatingWithin + 1 dormant coverage)
• Spec verifies RepeatingWithin < Within inheritance
• Spec verifies final_date/start_date match Within's computation
• StandardRB clean

Addresses QUAL-6317
Alternative to #71
Depends on #70 (EndOf fix)