Compile audience segment definitions into execution plans.
Partitions filters into SQL-evaluable and predicate-evaluable phases.

AND/OR/NOT composition of audience segment result sets.
FC/IS rule: pure functions only — no I/O, no side effects.

Well-formedness (unknown operator, NOT-inside-OR, circular/unknown/invalid
segment refs) is checked up front by `explain-composition`; the shell
validates a composition tree before evaluating it and raises the typed error
at the HTTP boundary. The evaluation functions below therefore assume a valid
tree and never throw — their error branches fail safe (an empty result set,
and the cycle guard still terminates).

Filter multimethods for audience segment evaluation.
FC/IS rule: pure functions only — no I/O, no side effects, no logging.

Validity is checked up front by `explain-filter` (pure, returns an anomaly
map or nil). The shell validates a segment's filters before compilation and
raises the typed error at the HTTP boundary, so the multimethods below assume
a known filter type and a supported operator and never throw — their unknown
branches fail safe (nil / a false predicate).

Pure Hiccup UI components for audience segments.
No I/O, no side effects.