spec: mutable hash tables (make-hash-table/ref/set!/delete!/etc)

Phase 10 — 11 primitives: make-hash-table, hash-table?, hash-table-set!,
hash-table-ref, hash-table-delete!, hash-table-size, hash-table-keys,
hash-table-values, hash-table->alist, hash-table-for-each, hash-table-merge!.
OCaml HashTable variant; JS Map-based. 28 tests, both hosts green.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

spec/primitives.sx

@@ -849,3 +849,5 @@
   :params ((a :as number))
   :returns "number"
   :doc "Number of bits needed to represent integer a (excluding sign).")
+
+(define-module :stdlib.hash-table)