fix: decontaminate M01 validation flow

[tim8es]

Purpose

Remove two confirmed sources of M01 cohort contamination without changing main, M02, or telemetry.

Fixes

• validation route no longer repeats the same cold Decision Drills already completed inside M01 Learning Lab
• post-case unlock now depends on submitted baseline + evidence-aware completion of both M01 lessons
• M01 cold drill first choice is frozen after feedback opens
• validation protocol requires a fresh browser context for each new participant

TDD

RED reproduced both defects: duplicate drill requirement and mutable first choice.
GREEN passed on the code head before the docs-only protocol update; PR CI on this final head is the release verification gate.

Scope

No main merge requested. No M02 activation. No telemetry changes.