Introduce bounds-based ranges, directional iterators, and Entry API updates; add docs/tests

[leaxoy]

Motivation

• Introduce a composable range abstraction and unify range/iteration APIs across ordered containers using explicit direction suffixes (Asc/Desc).
• Align Entry APIs across map-like types to a consistent upsert/delete contract that returns the previous value and existence flag, and add Delete helpers.
• Add repository-wide contributor guidance and agent instructions, and refresh README examples to reflect the new APIs.

Description

• Add a new bound package implementing Bound, RangeBounds and helpers (NewIncluded, NewExcluded, NewUnbounded) and Contains/IsValid semantics for comparator-based ranges, and add bound unit tests (bound/bound_test.go).
• Replace legacy range and iterator signatures with directional and bounds-based variants across ordered containers, e.g. Iter() -> IterAsc()/IterDesc(), Range(lower, upper) -> RangeAsc(bounds)/RangeDesc(bounds), and matching mutable variants like IterMutAsc/IterMutDesc and RangeMutAsc/RangeMutDesc (changes applied to btree, btreemap, btreeset, skipmap, skipset, etc.).
• Standardize Entry.Insert to return (old V, existed bool) and add Entry.Delete() for hashmap, btreemap, skipmap, trie and related entry implementations, and update underlying Insert/insert helpers to route through the Entry logic where applicable.
• API and utility updates: export bound aliases and helpers from ds.go (Bound, RangeBounds, NewRangeBounds), add GetComparator on BTreeMap, add Equal helpers on sets, update many iterators/Range callers in tests and code to use the new bound API, and refresh README.md, CONTRIBUTING.md, and AGENTS.md to document conventions.

Testing

• Reformatted code with gofmt -w as part of changes and updated examples and tests to the new APIs.
• Ran the unit test suite with go test ./..., and all automated tests succeeded.

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Codex Task