T-E3 — Before/after deviation comparator

[lightningpixel]

Epic: Epic E — Visual diagnostics
Priority: P2

Context

Lets a user see exactly what a mesh operation (e.g. decimate) changed, geometrically, rather than trusting it blindly.

Scope

• Colorized Hausdorff distance (pymeshlab) + a split-screen slider between versions.

Acceptance criteria

After a decimate, the deviation map shows where detail was lost.

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From ROADMAP_0.5.md.