feat(quests): gems ride on generated tray pieces, not pre-placed (#17)

[uhmin99]

User feedback: pre-placing every gem on the grid made stages swingy and kept goals small. Now gems spawn on generated tray pieces — assignGems attaches gems (~50% of pieces, big pieces sometimes two) drawing only colors still needed; a fully-collected color stops appearing; a placed gem is collected when a line through it clears; over-collecting is fine.

• Model: GameState.trayGems (per-slot cell->color) + gemGoal; placement.stampWithGems; engine threads gems through place/refill; quest controller passes the goal.
• Render: placed gems keep the piece color (pre-placed stay light); tray pieces show their gems (shared paintGemStar).
• Validator: dropped the obsolete 'board must contain the gems' rule, added a gem-count range. All 20 gem stages stripped of board gems, goals raised (single 8 / two 6 / three 5).

Tests: engine gem collection + needed-only refill + over-collect; assignGems needed-only/deterministic; trayGems/gemGoal save round-trip.

What & why

Checklist

• dart format . and flutter analyze are clean
• flutter test is green (new logic has new tests)
• Ran it on a device/emulator and exercised the affected flow
• One logical change; title is a valid conventional commit
• Nothing from the never-list (ads, IAP, analytics, tracking,
  locked content, extra network calls — see CONTRIBUTING.md)

Quest content only

• dart run tool/validate_quests.dart --update then
  dart run tool/validate_quests.dart prints OK
• Playtested the new stages (opening break feels good)
• I understand this ships to players as soon as dev is promoted
  to main (fast for content merges)

Screenshots