Numeric threshold defaults and A/B inventory cutoff

DEC-0021 - Numeric threshold defaults and A/B inventory cutoff

Context

After locking simple threshold architecture, need concrete numeric defaults so recovery/cooldown-reset behavior can run consistently.

Decision

Practice-time default thresholds:

• Reading: 8 minutes
• Listening: 8 minutes
• Writing: 12 minutes
• Speaking: 12 minutes

Completion-ratio default thresholds:

• Objective exercises: >= 70%
• Subjective exercises: >= 60%

Tier-C fallback cutoff:

• Tier C is allowed only when combined inventory of A+B candidates is less than 3.

Simplicity constraint:

• Keep these as compact global defaults for V2 baseline.
• Do not add extra branching layers unless data later proves need.

Rationale

This set keeps logic understandable for users and straightforward for implementation, while preserving recommendation quality and fallback safety.