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/sci/lang_stubs.clj

@@ -0,0 +1,32 @@
+; Minimal SCI type stubs for Jolt bootstrap
+; Provides the deftype definitions that SCI's Clojure source references
+; before the full SCI runtime is loaded.
+
+; Protocol stubs for SCI's type system
+(defprotocol IBox (setVal [this v]) (getVal [this]))
+(defprotocol HasName (getName [this]))
+(defprotocol IVar (bindRoot [this v]) (getRawRoot [this]) (toSymbol [this])
+  (isMacro [this]) (setThreadBound [this v]) (unbind [this]) (hasRoot [this]))
+(defprotocol DynVar (dynamic? [this]))
+(defprotocol IReified (getInterfaces [this]) (getMethods [this])
+  (getProtocols [this]) (getFields [this]))
+
+; Unbound sentinel for vars
+(deftype SciUnbound [the-var])
+
+; Core SCI types — keep minimal for bootstrap
+(deftype Namespace [name mappings aliases imports])
+(deftype Var [root name meta macro dynamic ns])
+
+; Store stub (sci.ctx-store provides a global context atom)
+(def ctx-store (atom nil))
+
+; Macro helpers from sci.impl.macros
+(defn deftime [& body] nil)
+(defn usetime [& body] (eval (first body)))
+(defmacro ? [& args]
+  (if (contains? &env (quote &env))
+    (let [form (first args)]
+      (if (= :clj (first form))
+        (second form)
+        (if (= :cljs (first form)) nil)))))