Architecture refactor: plan + phase 0 (dead code + bugs) (#106)

* Add architecture refactor plan

Synthesizes a six-part architectural review into phased, gate-validated cleanup
work. Targets LLM-maintainability: one home per feature, no god-files, explicit
checked contracts, no copy-paste dispatch. No code changes yet — the plan only.

* Refactor phase 0: dead code + isolated bugs

Pure cleanup ahead of the structural phases (docs/architecture-refactor-plan.md).
No behavior change except the two bug fixes, which are covered by a regression row.

Dead code (all verified zero-reference or overridden):
- core-resolve / core-satisfies? / core-type->str seed stubs + bindings —
  resolve and satisfies? are interned by install-stateful-fns! (the seed copies
  were shadowed); type->str was an inert SCI stub with no callers.
- find defined twice in 20-coll.clj; the dead copy returned a plain vector
  (wrong — the live def at :787 returns a real map-entry) with a comment that
  contradicted it.
- mark-hint (passes.clj), phs-to-struct (phm), shape-vals / ns-imports-fn
  (types) — unreferenced.
- redundant local pad2 in javatime (module-level one already in scope).

Bugs:
- File.toURL stored :url but every :jolt/url method reads :spec, so a URL from
  (.toURL file) returned nil from all its methods. Now stores :spec (+ spec row).
- pl-rest had a no-op (if (plist? r) r r); collapsed to r.
- :map-shapes? was missing from the deps-image cache key — two runs differing
  only in map-shapes could reuse each other's image.

Also dropped read-quote's unused pos param. Full gate green.

---------

Co-authored-by: Yogthos <yogthos@gmail.com>

test/spec/host-interop-spec.janet

@@ -107,6 +107,8 @@
    "(do (require (quote clojure.java.io)) (janet/spit \"/tmp/jolt-lineseq-spec.txt\" \"a\\nb\\n\") (vec (line-seq (clojure.java.io/reader \"/tmp/jolt-lineseq-spec.txt\"))))"]
   ["with-open closes shim" "97"
    "(with-open [r (StringReader. \"a\")] (.read r))"]
+  ["File.toURL methods read it back" "\"file:/tmp/x\""
+   "(do (require (quote clojure.java.io)) (.toString (.toURL (clojure.java.io/file \"/tmp/x\"))))"]
   ["vector :import shares deftype ctor" "\"hi!\""
    "(do (ns spec.nodea) (defprotocol SpecP (spec-pm [this])) (deftype SpecTN [t] SpecP (spec-pm [this] (str t \"!\"))) (ns spec.nodeb (:import [spec.nodea SpecTN])) (.spec-pm (SpecTN. \"hi\")))"])