📝 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.

backlog.tasks

@@ -42,10 +42,10 @@ Matcher:
 ✔ Clean up: adopt the 3-overload matcher (`src/prototype-ac2.ts`) and delete the prototypes @high @done
     → `getMatcher` / `getMatcherW`, each with overloads `ExhaustiveLoose` → `Fallback` → `Handlers` (order is load-bearing)
     → fold into `src/primitive.ts` / the public API; drop `src/prototype*.ts`
-☐ `_` should receive only the unhandled `T` keys, not all of `T` @medium
-    → today `_: (shape: T) => R`; desired `_: (shape: Exclude<T, handledKeys>) => R`
-☐ An exhaustive pattern that also carries `_` must be a compile error @medium
-    → `{ a, b, _ }` for `T = "a" | "b"` is accepted today; the redundant `_` should be rejected
+✔ `_` should receive only the unhandled `T` keys, not all of `T` @medium @done
+    → the fallback is now a second argument: `(handlers, (s) => …)`, `s: Exclude<T, keyof handlers>`
+☐ A fallback for an already-exhaustive handler map must be a compile error @medium
+    → `(handlers, fallback)` with `handlers` covering all of `T` is accepted; the redundant fallback should be rejected (currying would allow the guard)
 
 Bugs:
 ✔ TS 7 LSP server logs `context canceled` on stderr at shutdown @done