📝 Record the autocomplete and matcher-design decisions

The matcher is now two three-overload factories; library.md documents why
the union merge, inferred universe, conditional RequireKeys and cases-first
paths were rejected, and lists the two open issues (the fallback sees all of
T; a redundant _ is still accepted). testing.md records the language server
as the autocomplete oracle, and CONTRIBUTING points the exception at the
matcher's own test file.

The backlog marks the design-doc and autocomplete groundwork done alongside
the adoption.

backlog.tasks

@@ -27,6 +27,21 @@ v1.0:
     ☐ Achieve 100% branch coverage on `src/primitive.ts`
     ☐ Achieve 100% branch coverage on `src/index.ts`
 
+Testing:
+✔ Cover autocomplete with real completion test cases in the suite @medium @done
+    → `src/util/__tests__/lsp-completion.ts` (`#test-utils/…`) is the helper; the suite asserts its labels (see development/testing.md § Autocomplete)
+    → wire it into `node --test` so a test asserts the offered labels
+    → note: `Parameters<typeof factory>[0]` resolves only the *last* overload; use `@ts-expect-error` call sites for factory negatives, not `not.toExtend<Parameters<…>>`
+
+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)
+    → 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
+
 Bugs:
 
 Enhancements:
@@ -41,6 +56,7 @@ Documentation:
 ☐ Add comparison section vs. other TS pattern-matching libs in Readme.md
 ☐ Write migration guide for users coming from discriminated unions
 ☐ Create backlog tasks for implementation
+✔ Document the matcher design paths in `development/` — union vs overload merge, inferred universe (`NoInfer`), conditional `RequireKeys`, cases-first — and why each was abandoned @done
 ☐ Validate code fences in Markdown (start with README.md) — compile the TypeScript examples against `src/` so the docs cannot drift from the API
 
 Workflow: