📝 Document the two-argument fallback

Rewrite development/library.md around the new shape: the fallback is a second
argument (so it can receive the remainder), `R` needs an inference site in the
handler map, `Exact` restores the excess-property check, and the overload order
is load-bearing. Record this branch's rejected alternatives (single-object `_`,
currying, `this`/HKT, variance/`const`/`NoInfer`/brands) and the redundant-
fallback known issue. Add the changelog note and check off the backlog task.

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+- move the `_` fallback to a second argument typed `Exclude<T, keyof handlers>`, so it receives only the unhandled keys, and infer handler returns from short, unannotated handlers
+
 ## [0.3.0] - 2026-09-18
 
 - condensed primitive matcher factories down to 2 from formerly 4