Observer perspective controls (web)

[mbreiser]

Summary

Web-side counterpart to maDisplayTools #15 (Observer position & arena pitch).

Add observer perspective parameters to web tools:

• Arena pitch — tilt the arena relative to horizontal
• Observer translation — move observer position within arena (not just center)
• Observer height — vertical position of viewpoint

Design Questions

These match the MATLAB-side discussion (maDisplayTools #15):

• Are observer parameters part of arena config, pattern metadata, or runtime-only?
• Should observer position be saved with patterns or be a preview-only setting?
• How does this interact with the V2 header observer_id field?

Where to implement

• Pattern Editor — observer position controls in Generate tool
• Arena 3D Viewer — observer viewpoint adjustment
• arena-geometry.js — coordinate transformations accounting for observer position

Dependencies

• Coordinate with maDisplayTools Update icon generator status to 'In Development' with v0.3 #15 for consistent design decisions
• Spherical geometry is fully implemented (arena-geometry.js) — this builds on that foundation