compiler: compile set?/disj as plain fns (close the last Stage-1 fallback gap)

Stage 1 jolt-g3h. set? and disj were special-cased in all three "can't compile"
lists (host_iface special-names, compiler.janet uncompilable-heads, evaluator
special-symbol? + handlers) — but they're pure value-production with callable
core vars (core-set?/core-disj), and those vars are byte-for-byte equivalent to
the evaluator handlers. Removed them from all three lists + dropped the now-dead
evaluator handler arms, so they're ordinary clojure.core fns everywhere: the
analyzer compiles (set? x)/(disj s x) as normal var calls instead of punting to
the interpreter.

Verified identical results in default AND JOLT_MUTABLE builds (no representation
sensitivity — sets are phs in both, unlike vector?/list? which collapse).

With this, the self-hosted analyzer's compile-path fallback set equals the frozen
intentional stateful set (Task 2) — it's now a strict superset of the bootstrap
compiler's compilable surface, so the Janet bootstrap is retireable (Stage 2).

fallback-zero: set?/disj moved to must-compile (31 now), set! into must-punt.
Gate: conformance 267x3 (+5 set?/disj cases), lazy-infinite 44/44, suite 4034/67,
fixpoint, self-host, sci, specs+unit green.

test/integration/conformance-test.janet

@@ -56,6 +56,12 @@
    ["empty set literal"    "true" "(empty? #{})"]
    ["set literal count"    "3"    "(count #{1 2 3})"]
    ["set literal in let"   "true" "(let [x 5] (= #{5 6} #{x (inc x)}))"]
+   # set?/disj compile as plain fns now (jolt-g3h), not special forms
+   ["set? true"   "true"      "(set? #{1 2 3})"]
+   ["set? false"  "false"     "(set? [1 2])"]
+   ["disj one"    "#{1 3}"    "(disj #{1 2 3} 2)"]
+   ["disj many"   "#{1}"      "(disj #{1 2 3} 2 3)"]
+   ["disj absent" "#{1 2}"    "(disj #{1 2} 5)"]
    ["keyword as fn" "1"   "(:a {:a 1})"]
    ["map fn over coll" "(quote (1 3))" "(map {:a 1 :b 3} [:a :b])"]