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.

CLAUDE.md

@@ -86,7 +86,7 @@ reader/analyzer/IR/backend (`jolt-core/jolt/`) and `clojure.core` in
 dependency-ordered tiers (`jolt-core/clojure/core/NN-*.clj`, loaded in order:
 00-syntax, 00-kernel, 10-seq, 20-coll, 25-sorted, 30-macros, 40-lazy, 50-io).
 The stdlib namespaces (`clojure.string`/`set`/`walk`/`edn`/`pprint`/…) are
-portable Clojure under `src/jolt/clojure/`.
+portable Clojure under `stdlib/clojure/`.
 
 `bin/joltc` (`host/chez/cli.ss`) loads the checked-in seed
 (`host/chez/seed/{prelude,image}.ss`) + the spine and compiles+evals on Chez
@@ -114,3 +114,19 @@ Issue tracking and design notes live in beads (`bd prime`, `bd memories`).
   is the contract.
 - **Gate every change**: `make test` with a real exit code (self-host fixpoint,
   corpus floor, unit, cli smoke, certify). Re-mint if a seed source changed.
+
+## Writing style (comments, docstrings, docs)
+
+Write like a human maintainer of a serious open-source project. Plain, terse,
+factual. Document how the code works *now* — what it does and why it matters.
+
+- No LLM tells: drop "Note that", "It's worth noting", "Importantly", "simply",
+  "essentially", "in order to", "under the hood", and marketing words
+  ("comprehensive", "robust", "seamless", "leverage", "powerful").
+- No historical exposition (how the code used to work, porting notes, "the prior
+  X"), no internal issue IDs (`jolt-xxxx`) or milestone tags ("Phase N") in
+  comments or docstrings. The git history and beads hold that.
+- Keep genuine semantic contrasts with JVM Clojure — those document real,
+  user-visible behavior.
+- Don't restate the code; explain the non-obvious. Match the surrounding file's
+  comment density and tone.