T-H1 — Declare an asset's real-world scale

[lightningpixel]

Epic: Epic H — Real-world scale & measurement tools
Priority: P2

Context

Without a calibrated scale, an export comes out at an arbitrary size — "4 meters or 4 millimeters", per IDEAS.md. Critical for 3D printing (Epic D) and, eventually, AR. Verified state: there is no real-scale or measurement-tool concept anywhere in the code (Viewer3D.tsx, optimize.py) — generated meshes only carry the arbitrary scale that came out of the AI generator.

Scope

• set_real_scale op in the registry (T-A1): "this object is 30 cm" → computes the scale factor from the current bbox and applies it via transform_mesh.
• Reuses normalize_scale (T-A3) as an internal building block instead of duplicating the logic.

Acceptance criteria

Declaring a dimension recalibrates the mesh, and every later export (T-D2) comes out at the correct units.

Depends on: T-A1, T-A3

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From ROADMAP_0.5.md.