♻️ Name the matcher factories getMatcher / getMatcherW

Drop the strict/widened aliases from src/index.ts so the factories are
exported under their own names, and align the type tests, the autocomplete
probe (which now imports the public surface from ./index.ts) and library.md
with them.

Also correct the widened-overload comment: the excess-property check does not
apply to a generic P, so MatcherWidening's keyof guard — not the closed
constraint — is what rejects keys outside T/_, as library.md already
documented.

backlog.tasks

@@ -35,7 +35,7 @@ Testing:
 
 Matcher:
 ✔ Clean up: adopt the 3-overload matcher (`src/prototype-ac2.ts`) and delete the prototypes @high @done
-    → `strict` / `widened`, each with overloads `ExhaustiveLoose` → `Fallback` → `Handlers` (order is load-bearing)
+    → `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`