api-stability.md: defines stable/experimental/deprecated API tiers for
v0.1.x. Stable: Engine.solve, registries, PerformanceProfile fields.
Experimental: individual solvers, pipeline, DSE, CLI.
understanding-efficiency.md: reframes the eta parameter as a teaching
opportunity (CPI analogy from Patterson & Hennessy). Enumerates all 12
sub-factors, provides calibration procedure, and documents 4 fallacies.
