Return Optional[T] from all partial-result terminals (BREAKING) (#42)

Unify every "maybe no result" terminal on Optional, replacing the
(value, bool) / (int, bool) / (key, value, bool) returns. This is a
breaking change, made now while the library has no downstream importers.

  - Seq:   Find, FindLast, First, Last, Nth, Reduce, MaxBy, MinBy,
           MaxByKey, MinByKey  -> Optional[T]
           FindIndex, FindLastIndex                    -> Optional[int]
  - free:  Max, Min -> Optional[T];  IndexOf, LastIndexOf -> Optional[int]
  - subtypes: SeqOrdered/SeqNumeric Max/Min/IndexOf, SeqComparable.IndexOf
  - Seq2.Find -> Optional[Pair[K, V]] (key/value wrapped in the existing Pair)

Optional.Get() (T, bool) is retained as the bridge back to Go's idiomatic
pair; ToOptional(v, ok) bridges the other way. Pure-bool predicates
(Any/All/None/Contains/Equal/IsEmpty) and the FilterMap callback signature
func(T)(U,bool) are deliberately untouched.

Docs (README, README_CN, docs/index.html) updated to the new signatures
and chaining examples. Also adds docs/index.html (full API reference) and
removes the stale docs/issue#0001.html–0010.html scratch files.

Closes #36
Closes #37
Closes #38
Closes #39
Closes #40
Closes #41

Co-authored-by: chaoyuepan <chaoyuepan@baidu.com>

Use go1.27rc1 in CI; add badges to READMEs