Observable (map / filter / take)

Implement a minimal synchronous Observable class:

• Observable.of(*values) — static factory from a sequence of values.
• obs.map(fn) — returns a new Observable applying fn to each value.
• obs.filter(fn) — returns a new Observable keeping values where fn is truthy.
• obs.take(n) — returns a new Observable with at most n values.
• obs.subscribe({'next': fn}) — runs the chain synchronously, calling fn for each value.

solve applies a declarative operator chain to a list of values.
Operators: ['map', expr], ['filter', expr], ['take', n] where
expr is a lambda string compiled with eval.