core: AOT context image — init-cached recovers the bootstrap cost across processes

init in compile mode is ~2.4 s (tier loading, analyzer self-compile, macro
recompilation), paid by every process that builds a ctx from source — each
jpm-test file, embedders, workers. init-cached marshals the built ctx to a
disk image (same root-env dicts as snapshot/fork) and later processes
unmarshal it in ~5 ms, any process: nothing from the baking process is
needed at load.

The cache key fingerprints the embedded .clj stdlib (which covers jolt-core:
analyzer, IR, core tiers), the .janet seed sources next to the module, the
janet version, the init opts, and the env knobs that shape a ctx (JOLT_PATH/
MUTABLE/AOT_CORE/FEATURES) — any change rebuilds. Corrupt or non-ctx images
fall back to a rebuild (unmarshal of garbage can 'succeed' with a scalar, so
the shape is checked, not just the throw). Writes are atomic (tmp + rename)
so racing cold starts never publish a torn image. JOLT_NO_IMAGE_CACHE=1
opts out; JOLT_IMAGE_CACHE_DIR overrides the location (default TMPDIR).

Test consumers switch to init-cached (harness, suite-worker, conformance,
the behavioral unit/integration tests); tests that validate the bootstrap
itself (bootstrap-fixpoint, staged-bootstrap, aot round-trip, direct-linking)
and the deps tests (tmp-dir :paths would fragment the key) keep real init.
Full jpm test: 2:46 -> 1:58 (~29%). New ctx-image-test covers cold/warm,
cross-process load (subprocess runs defn/redef/macros/protocols/multimethods
off the baked image), per-opts keying, and corrupt-image fallback.

test/integration/api-test.janet

@@ -2,7 +2,7 @@
 (use ../../src/jolt/types)
 
 (print "1: init creates context...")
-(let [ctx (init)]
+(let [ctx (init-cached)]
   (assert (ctx? ctx) "init returns context")
   (let [ns (ctx-find-ns ctx "clojure.core")]
     (assert (ns? ns) "clojure.core namespace exists")
@@ -11,27 +11,27 @@
 (print "  passed")
 
 (print "2: eval-string basics...")
-(let [ctx (init)]
+(let [ctx (init-cached)]
   (assert (= 42 (eval-string ctx "42")) "eval integer")
   (assert (= true (eval-string ctx "true")) "eval bool")
   (assert (= 3 (eval-string ctx "(+ 1 2)")) "eval list"))
 (print "  passed")
 
 (print "3: eval-string with core fns...")
-(let [ctx (init)]
+(let [ctx (init-cached)]
   (assert (= true (eval-string ctx "(nil? nil)")) "nil?")
   (assert (deep= [2 3 4] (normalize-pvecs (eval-string ctx "(map inc [1 2 3])"))) "map+inc")
   (assert (= 6 (eval-string ctx "(reduce + [1 2 3])")) "reduce"))
 (print "  passed")
 
 (print "4: eval-string with def...")
-(let [ctx (init)]
+(let [ctx (init-cached)]
   (eval-string ctx "(def x 42)")
   (assert (= 42 (eval-string ctx "x")) "def then resolve"))
 (print "  passed")
 
 (print "5: eval-string* with bindings...")
-(let [ctx (init)]
+(let [ctx (init-cached)]
   (assert (= 99 (eval-string* ctx "y" @{"y" 99})) "bound variable"))
 (print "  passed")