[B5] Advanced progression: per-domain levels, leaderboard seasons, multi-cadence streaks

Backlog. Extensions to the progression system once the MVP is calibrated in the wild.

Depends on: [G1] XP ledger, [G7] Leaderboard.

Ideas parked here

Per-domain levels alongside the global one (cinephile 4 / reader 2 / player 1). Gives an RPG-like "skill" dimension, lets someone dominate one domain without leading overall, and keeps the global level from flattening identity. Recomputable, but harder — the XP has to be attributable per domain from the start, which is worth checking in [G1].
Leaderboard seasons with an archived podium and an achievement for placing. Prevents early adopters from locking the top forever, and creates a recurring event. Recomputable from the ledger, so the MVP (month/year filters) does not close this door.
Multi-cadence streaks. A daily streak only really makes sense for video; for books and games the honest unit is the week. One mechanic, two cadences depending on the domain. Calibration is the hard part and is not obvious — parked deliberately.

Real playtime is not measured outside the Steam import, which limits any game-based cadence.