📝 Restore and rewrite the README entry point

The README collapsed to API-only prose once the old match/P surface was
dropped. Rebuild its entry-point shape: a quick-start Synopsis walked through
a Contact union, a dedicated Installation section, and a real-world Examples
section (primitive-union dispatch, a fallback, a property union narrowed by
the tagged-union matcher, and the widening variant).

Set the tagline and Description to the library's identity — exhaustive,
type-safe pattern matching for TypeScript — and state the goal Moose-style:
type-safe pattern matching with a lean syntax, accomplished by exhaustive
branches and typed per-branch handler parameters, backed by autocomplete and
a tiny footprint. Drop the F#-style framing and the stale "matches method is
a type guard" and "(not regex)" copy from the README, AGENTS.md and the
package.json description. Every fence is a doc-test, so the examples cannot
drift from the API.

backlog.tasks

@@ -23,7 +23,7 @@ Matcher:
     ✔ optional discriminant (`{ type?: "x" }`) is the same hole: the boolean/nullish change now admits the `undefined` tag, so the factory accepts the key, but `Extract<T, Record<K, V>>` still passes `never` to both the `x` and `undefined` handlers @done
 
 Documentation:
-☐ Bring README.md back to its previous form — synopsis and examples restored, in the correct place
+✔ Bring README.md back to its previous form — synopsis and examples restored, in the correct place @done
     → previous section order: title, tagline, Synopsis, Description, Requirements, Examples, API, License, Contributing
     → previous Examples order: literal/exhaustive, typeof, structural/discriminated unions, when, any
 ☐ Create `examples/` directory with runnable snippets