docs(readme): cut 2025-only / deprecated walkthroughs; simplify #49

Job	Run time
Spec mutex (both features must not compile)	17s
Default lane (2026-07-28)	6m 11s
Rustdoc (deny warnings)	1m 0s
Opt-in lane (2025-11-25)	5m 40s
	13m 8s

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