docs: add ADR 0001 for the per-collection module rewrite

Records the decisions behind the rewrite of the collection modules: one
folder and signature file per collection, `FrozenMap`/`FrozenSet`
support, `voption` and struct-tuple defaults with `Option`/`Tuple`
override namespaces, category-based function order, BCL/LINQ-first
implementation, shared contract tests, one pull request per collection,
and the function-by-collection support matrices.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: xperiandri