Simplify error handling with orthogonal return semantics

Breaking API change for pre-1.0.0: the returned function now controls
flow (continue/stop) while the returned error is data passed to handlers.

Removed Continue() wrapper in favor of checking function nilness. Info
gains Next and Err fields. Handler.Handle now takes only Info. Do checks
f == nil && i.Err != nil for fatal path. Log uses i.Next to distinguish
non-fatal errors (⊘) from fatal ones (✘).

Simplify Do

Fix Equal to compare fully qualified function names

Equal now uses the full runtime name instead of the short name to
prevent false positives when comparing identically named functions
from different packages. Name still returns the short form for display.

Initial commit