feat(yggdrasil): add lightweight town crowds

[CloudeaSoft]

Summary

Replace the basic town crowd VFX with lightweight travelers that navigate Yggdrasil Town without consuming native NPC slots. Travelers arrive through the giant Original Pylon, visit town locations, converse, sit, board trains, and leave through supported destinations.

• Add randomized appearances and names with hover text, peer conversations and emote bubbles, seating, and building entrances.
• Support train boarding, riding, and alighting, with passengers rendered behind the carriage shell.
• Add threat avoidance and combat-area spawn suppression. A shared, reversible fade-out recycles travelers after sustained danger, stalled movement, or repeated departure failures while preserving their ongoing activities.
• Keep player configuration limited to the crowd toggle and density, with English and Simplified Chinese localization.
• Organize the implementation under Mechanics/TownCrowd, remove the legacy crowd controller and test item, and simplify redundant interfaces.

Validation

• dotnet build /p:WarningLevel=0 --no-restore succeeded.
• All 162 Crowd-related tests passed.
• Byte-level UTF-8 BOM check against origin/1.4.4 passed.
• Existing ImageSharp vulnerability warnings and the test-discovery warning for missing FontStashSharp.FNA remain.

Final in-game verification is still pending for overlapping retirement causes, recovery during fading, pylon/carriage opacity composition, and conversation/seat cleanup after retirement. No multiplayer support is introduced; crowds remain restricted to single-player Yggdrasil.

Closes #432