STATUS: stub — full derivations land in the pedagogy pass (spec section 7, foundations scope ~ Faist lecture notes). Lean L1 target: this topic's framework definitions (spec section 11).
One-liner: Syndrome decoding is hypothesis testing on cosets; matching and BP are its workhorses.
Planned sections: Essence / Design problem / Derivation chain / Multiple pictures / Why these parameters / Failure modes / Operations story / Litmus questions.