⚡ Index dispatch with the raw shape key

String(shape) defeats V8's numeric-key fast path (measured ~2x on
number-keyed dispatch). JS already coerces boolean/null/undefined to the
same property key, so assert shape to string | number and index directly.
The assertion is TS2538-only; the facade keeps it sound. Needs a source
no-unsafe-type-assertion disable, next to the existing one.

development/library.md

@@ -106,9 +106,11 @@ offers `true`, `false`, `null`, `undefined` by name (verified over LSP).
 
 #### Why
 
-- Runtime dispatch is unchanged in effect: `handlers[true]` already coerces to
-  `"true"`. `dispatch` wraps the index in `String()` only because TypeScript
-  forbids indexing with `boolean`/`null`/`undefined` (`TS2538`).
+- Runtime dispatch indexes with the raw `shape`; `handlers[true]` coerces to
+  `"true"` at runtime exactly as `String` would. The `shape as string | number`
+  assertion only placates `TS2538` and buys the number fast path (an explicit
+  `String()` defeats V8's numeric-key path: measured ~2× on number-keyed
+  dispatch).
 - `symbol`/`bigint`/`NaN`/`-0` are rejected, and a member colliding with its
   stringification is unguarded: user-facing, stated once in
   [README § Caveats](../README.md#caveats).