docs(zset): the reasoning, the evidence and the wasm gate entry (improvise-tk51)

Registers zset in nix/ci.nix's wasmCrates and the doc site, shortens the
comment runs, and fixes the clippy findings in the test crate.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

nix/ci.nix

@@ -48,6 +48,8 @@
     "improvise-gp"
     # ADR 005 clause 14: nothing depends on it yet, and improvise-core will.
     "numtower"
+    # ADR 009 clause 1: std-only; nothing depends on it yet.
+    "zset"
   ];
   pFlags = crates: lib.concatMapStringsSep " " (c: "-p ${c}") crates;