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

scripts/build-doc-site.mjs

@@ -68,6 +68,7 @@ const SECTIONS = [
       ["context/code/cli.md", "code/cli", "The headless CLI"],
       ["context/code/gp.md", "code/gp", "gp-tree and improvise-gp"],
       ["context/code/numtower.md", "code/numtower", "numtower, the numeric tower"],
+      ["context/code/zset.md", "code/zset", "zset, the Z-set group"],
       ["context/code/kb-tool.md", "code/kb-tool", "The kb tool"],
       ["context/code/tooling.md", "code/tooling", "The repository's own checkers"],
     ],
@@ -87,6 +88,7 @@ const SECTIONS = [
       ["context/test-evidence/cli.md", "test-evidence/cli", "The headless CLI"],
       ["context/test-evidence/gp.md", "test-evidence/gp", "gp-tree and improvise-gp"],
       ["context/test-evidence/numtower.md", "test-evidence/numtower", "numtower"],
+      ["context/test-evidence/zset.md", "test-evidence/zset", "zset"],
       ["context/test-evidence/tooling.md", "test-evidence/tooling", "The repository's own gates"],
       ["context/test-evidence/layout-cost.md", "test-evidence/layout-cost", "Layout and keypress cost"],
       ["context/test-evidence/scale-bisect.md", "test-evidence/scale-bisect", "Scale bisect"],