core: Stage 2 tier 6c — dispatch-table ops + misc compile (macros/plain invokes)

prefer-method/remove-method/remove-all-methods/get-method/methods become
overlay macros over ctx-capturing *-setup fns (a multimethod's method table
lives on its VAR, so the name passes quoted — the defmulti/defmethod shape).
instance? likewise (class names don't evaluate to values); satisfies? is a
plain ctx-capturing fn (evaluated args). locking and defonce become overlay
macros — locking now also evaluates the monitor expr (the old arm skipped it
and any body form past the first); defonce keeps the existing-root check.
read-string and macroexpand-1 are ctx-capturing fns.

Removed all eleven from the evaluator special arms + special-symbol?,
host_iface special-names, and compiler uncompilable-heads. evaluator-test's
locking/instance? cases use init now (overlay macros need the full env).

Surfaced pre-existing (filed): multimethod dispatch records prefer-method
preferences on the var but never consults them in ambiguous isa dispatch.

Gate: conformance 296x3 (+11 tier-6c cases), fallback-zero 73/3, fixpoint,
self-host, sci, staged, suite 4049>=4034, all specs+unit.

test/integration/fallback-zero-test.janet

@@ -64,7 +64,13 @@
    "(the-ns (quote clojure.core))" "(ns-interns (quote clojure.core))"
    "(ns-aliases (quote user))" "(ns-imports (quote user))"
    "(ns-resolve (quote clojure.core) (quote map))" "(resolve (quote map))"
-   "(refer (quote clojure.string))"])
+   "(refer (quote clojure.string))"
+   # Stage 2 tier 6c: dispatch-table ops + misc compile as macros/plain invokes
+   "(prefer-method mf :a :b)" "(remove-method mf :a)" "(remove-all-methods mf)"
+   "(get-method mf :a)" "(methods mf)"
+   "(satisfies? P 5)" "(instance? String \"x\")" "(locking :x 1)"
+   "(defonce fz-once 1)" "(read-string \"[1 2]\")"
+   "(macroexpand-1 (quote (when true 1)))"])
 
 # --- Intentional fallback (sanity sample): these SHOULD punt to the interpreter.
 # The remaining frozen/uncompiled set keeps the harness honest in the punt