Clean up codebase: rename stdlib layer, strip porting residue, fix tooling

Rename src/jolt -> stdlib (the runtime-loaded layer; jolt-core stays the
seed-baked layer) and update the loader / emit-image / doc paths. Drop dead
code: the spike/ experiments, the duplicate clojuredocs-export.edn (json moves
to tools/), the Janet-era jolt.http binding, and the orphaned
persistent_vector.clj whose ns/path didn't even match.

Strip porting residue from comments and docstrings across host/chez, jolt-core,
stdlib, tests, and docs: internal issue ids, "Phase N" markers, and the "vs
Janet" historical exposition, leaving present-tense descriptions and the real
JVM-Clojure semantic contrasts. Same pass over the corpus suite labels. The seed
is unchanged (docstrings/comments aren't emitted), so the self-host fixpoint and
corpus are untouched.

Port tools/spec_coverage.py off the dead janet probe to bin/joltc and regenerate
coverage.md; drop the dead :host/janet rule from certify.clj and regenerate the
conformance profile. Add docs/host-interop.md (the JVM shims and how to register
your own host class from a library) and a writing-style note in CLAUDE.md.

Stabilize the four racy concurrency corpus cases (future-cancel and agent
send/send-off): give the future a sleeping body and the agent a slow action, so
cancel reliably catches an in-flight future and deref reliably reads the
pre-update snapshot. They certify deterministically now, so drop their :flaky
allowlist entries and the orphaned legend.

stdlib/clojure/template.clj

@@ -0,0 +1,33 @@
+;; Verbatim from clojure.template (Stuart Sierra) — pure Clojure over
+;; clojure.walk, which jolt ships. Added so honeysql's :clj branch (which
+;; requires clojure.template) loads under JOLT_FEATURES including clj.
+(ns clojure.template
+  "Macros that expand to repeated copies of a template expression."
+  (:require [clojure.walk :as walk]))
+
+(defn apply-template
+  "For use in macros.  argv is an argument list, as in defn.  expr is
+  a quoted expression using the symbols in argv.  values is a sequence
+  of values to be used for the arguments.
+
+  apply-template will recursively replace argument symbols in expr
+  with their corresponding values, returning a modified expr.
+
+  Example: (apply-template '[x] '(+ x x) '[2])
+           ;=> (+ 2 2)"
+  [argv expr values]
+  (assert (vector? argv))
+  (assert (every? symbol? argv))
+  (walk/postwalk-replace (zipmap argv values) expr))
+
+(defmacro do-template
+  "Repeatedly copies expr (in a do block) for each group of arguments
+  in values.  values are automatically partitioned by the number of
+  arguments in argv, an argument vector as in defn.
+
+  Example: (macroexpand '(do-template [x y] (+ y x) 2 4 3 5))
+           ;=> (do (+ 4 2) (+ 5 3))"
+  [argv expr & values]
+  (let [c (count argv)]
+    `(do ~@(map (fn [a] (apply-template argv expr a))
+                (partition c values)))))