📝 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.

CONTRIBUTING.md

@@ -67,6 +67,11 @@ before the implementation. The loop is **type → red → green → refactor**:
    `npm run verify` as the definition-of-done gate.
 
 Every test pairs an `expectTypeOf(...)` with an `assert.*`; keep them together.
+The autocomplete tests (`src/util/__tests__/lsp-completion.test.ts` for the
+helper, `src/primitive.test.ts` for the matcher's popup) are the
+exception —
+the language server, not the type system, is the oracle (see
+[development/testing.md § Autocomplete](./development/testing.md#autocomplete)).
 Each test body follows **AAA (Arrange–Act–Assert)** with labeled blocks
 separated by a blank line: `// Arrange` sets up the inputs (e.g. the matcher
 factory), `// Act` exercises the subject once from them (not a second