feat: add exact wall-length input during drafting

[aryansk]

Summary

• Add an exact-length input during wall drafting in both the 2D and 3D flows.
• Parse the entered measurement with the existing unit-aware wall measurement behavior.
• Keep the live wall measurement and unit toggle behavior aligned with the entered value.

Validation

• 596 editor tests
• Editor typecheck
• Nodes, core, and viewer builds
• Biome
• git diff --check

Closes #308