Tier-aware difficulty: auto-detect lowerTierAdversary and highlight off-tier adversaries in builder

[gwillish]

Summary

Once player levels are captured (#70) and DHModels exposes tier utilities (gwillish/DHModels#21), the difficulty assessor and adversary roster should become tier-aware.

Blocked by

• Add level field to PlayerForm and PlayerStore #70 (player level capture)
• Add level to Player/PlayerConfig; add tier utilities and auto-detect lowerTierAdversary in DifficultyBudget DHModels#21 (tier utilities + auto-detect in DifficultyBudget)

Party tier calculation

Party tier = DifficultyBudget.partyTier(levels:) — median character level → tier, rounding up when the median straddles a tier boundary.

Examples:

Party levels	Median	Party tier
[1]	1	T1
[2, 3, 4, 5]	3.5 → rounds up → 4 → T2	T2
[4, 5]	4.5 → rounds up → 5 → T3	T3
[7, 8, 9]	8	T4

Changes

DifficultyAssessorView

• Compute party tier from playerConfigs.map(\.level) and pass to DifficultyBudget.suggestedAdjustments
• lowerTierAdversary auto-detected (and shown as auto-applied) when any roster adversary is below party tier
• Display party tier alongside the budget summary, e.g. "Party: T2 · 4 players · 14 pts"

AdversaryRosterRow / AdversaryRosterSection

• Highlight adversaries whose tier differs from party tier:
  • Below party tier — amber tint + "T1" badge (still addable; just flagged)
  • Above party tier — red tint + tier badge
  • Matching tier — no decoration
• Tooltips / accessibility hints should describe the tier relationship

Notes

• Highlighting is informational only — no adversary is blocked or removed
• With enough lower-tier adversaries the budget will still show the encounter as challenging (the budget numbers tell that story)
• If all playerConfigs have level == 0 or the array is empty, skip tier highlighting gracefully (no party tier derivable)