Complete measurable tag leader geometry

[epowell40]

What changed

• converts newly created geometry-aware tags to a supported free leader endpoint before native readback
• records exact end-condition, endpoint, segment, and collision receipts
• protects existing attached leaders with a conservative tagged-element-to-elbow/head corridor instead of ignoring unavailable attachment endpoints
• preserves fail-closed removal for unsupported or unmeasurable created leaders

Why

Revit does not expose GetLeaderEnd for attached leaders. The prior implementation therefore removed otherwise-clear Mechanical Equipment Tags because actual leader geometry could not be proven, and dense views stopped on existing attached leaders.

Validation

• 61/61 focused tag tests
• 208/208 RevitBridge.Common tests
• Revit 2024 and 2025 logic builds: 0 errors
• Revit 2024 add-in shell build: 0 errors
• Snowdon live apply/readback/GUI/cleanup: tag 1548112 on HRU 1465348, actual 3.043488 ft leader, zero collisions/crossings, exact cleanup verified
• original dense L4 attached-leader dry run no longer throws; conservative corridor rejects an unsafe forced duplicate