Deprecate Split2 and OrderedSplit2

[destel]

Since the introduction of Tee in v0.8, splitting no longer needs a dedicated operation — it can be composed from existing ones, and the composition isn't limited to two branches.

When the predicate is a simple pure check, splitting is just Tee + Filter on each branch:

adults, minors := rill.Tee(users)
adults = rill.Filter(adults, 1, func(u User) (bool, error) { return u.Age >= 18, nil })
minors = rill.Filter(minors, 1, func(u User) (bool, error) { return u.Age < 18, nil })

When the predicate is expensive, stateful, or can fail, evaluate it once per item before Tee, tagging each item with the decision and routing on the tag:

type Decision struct {
    Value    User
    Decision bool
}

tagged := rill.Map(users, n, func(u User) (Decision, error) {
    ok, err := isAdult(u)
    return Decision{Value: u, Decision: ok}, err
})

tagged1, tagged2 := rill.Tee(tagged)
adults := rill.FilterMap(tagged1, 1, func(d Decision) (User, bool, error) { return d.Value, d.Decision, nil })
minors := rill.FilterMap(tagged2, 1, func(d Decision) (User, bool, error) { return d.Value, !d.Decision, nil })

The same pattern extends to n-way splitting by tagging with an index or key instead of a bool.

The deprecated functions are now implemented as exactly this composition, so the existing Split2 tests validate the migration path until removal in v1.0.

Also mentions the upcoming v1.0 removal in DrainNB's deprecation notice.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.25%. Comparing base (f825caa) to head (c9e840f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
- Coverage   99.28%   99.25%   -0.04%     
==========================================
  Files          15       15              
  Lines         697      667      -30     
==========================================
- Hits          692      662      -30     
  Misses          5        5              

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