P0: deck-analysis scaffold + deckStats.ts

[Doberjohn]

Create the apps/web/src/features/deck/analysis/ module and implement deckStats.ts.

Scope — calculateDeckStats(deck, getCardById) returns: total cards, unique cards, ink distribution, cost curve (0..6+ buckets), type distribution, inkable count, ink count, legality (≥60, ≤4 per unique fullName, ≤2 inks via canShareDeck/getInks) + enumerated legalityErrors.

• Pure, no React. Handle unknown/rotated cardIds gracefully.

Tests (5–15): legal 60-card deck, under-60, 5th copy, 3rd ink, dual-ink counted as both.

Part of #450 · Deck Builder & Engine Score milestone.

[Doberjohn]

Delivered and merged into the deck-builder integration branch via #480 (not yet in production / master). Closing as complete; it rides to production when deck-builder later merges to master.