Product: metrics and mechanisms, judged on the real product

What this subtree is for. The product syllabus queue: A-line metrics and mechanisms (retention curves, north star, LTV, activation, AARRR, network effects, experiment statistics), B-line pricing and strategy mechanisms, C-line frameworks compressed. Each entry is a closed, testable item: a mechanism plus its cause, not a template recital.

Working rules for this subtree.

• Every item lands on his real products: Lucerna first (pricing ¥18/¥180, energy-bar quotas, activation and retention, north-star candidates), StandMeet as the artwork-type contrast. No fictional company cases.
• Depth parity with the SWE fundamentals shelf: a formula comes with its derivation, a number with its origin. Shallow framework recital is not accepted.
• Mechanisms first, templates compressed: C-line items are covered for construction only.

Queue status. A-line item 1, retention curves, done (see retention-curve). Next: north star.