Publish general counterfactual M2 compatibility

[Legender134]

Summary

• publish the exact allowlisted snapshot of the M2 compatibility compiler
• add spatialcf/planar_translate@2 compatibility contracts
• add the thin backend that delegates to the existing v2 solve owner

Snapshot authority

• development commit: d1d6dcbf509067af8016607c30d2233e86c9d52e
• public file count: 150
• public tree SHA-256: 496fd574c5c1f7a1bb4a3ac604fe196ed56fb3cf53271307f7a2750d42d58d19
• exact public diff: three additive source modules

Verification

• verified public Git worktree against the private manifest
• public smoke: 39 passed
• wheel and sdist build: passed
• development merged-main CPU gate: parallel 3318 passed, 56 skipped; serial 183 passed, 3378 deselected