feat: complete atom implementation, new macros, predicates, and test coverage

## Runtime (src/jolt/core.janet, +637/-99)

### Atom system (complete)
- core-atom: :validator and :meta options, watches table, atom-validate/atom-notify-watches helpers
- New functions: swap-vals!, reset-vals!, compare-and-set!, set-validator!, get-validator, add-watch, remove-watch
- Validator: blocks invalid reset!/swap!; watches: notification with old/new values; metadata support
- All 7 functions registered in core-bindings

### New predicates
- integer?, boolean?, list? — all registered in core-bindings

### New math
- abs, rand, rand-int — registered in core-bindings

### Control flow macros (7 new)
- cond, case (with one-of-many list support), condp, dotimes, while, if-not, when-first
- Registered as macros: cond, case, for, dotimes, while, condp, if-not, when-first

### Threading macros (7)
- -> (thread-first), ->> (thread-last), some->, some->>, cond->, cond->>, as->
- Renamed to core-thread-first/core-thread-last to avoid collision with comparison operators

### for comprehension
- Full Clojure for macro: :when, :let modifiers, nested bindings
- Uses map/mapcat expansion, correct last-group optimization

### Sequence operations (15 new)
- last, butlast, second, ffirst, fnext, nfirst, nnext, nthnext, nthrest
- drop-last, take-nth, map-indexed, keep, keep-indexed, reductions

### Nil-safety fixes
- core-reverse, core-sort, core-sort-by, core-distinct now handle nil input

## Tests (7 new files, 361 assertions across all tests)

### Ported from Clojure test suite (4 files)
- clojure-logic-test.janet: 82 assertions (if/and/or/not/some?/nil-punning)
- clojure-control-test.janet: 56 assertions (do/when/if-let/cond/dotimes/while/loop/case)
- clojure-macros-test.janet: 21 assertions (->/->>/some->/some->>/cond->/cond->>/as->)
- clojure-for-test.janet: 4 assertions (:when/:let/nested for)

### Systematic coverage
- clojure-atom-test.janet: 31 assertions (creation/reset!/swap!/swap-vals!/reset-vals!/CAS/validator/watches/meta)
- systematic-coverage.janet: 120+ assertions (predicates/math/comparison/logic/collections/seq-ops/destructuring)
- logic-test.janet: earlier ported logic tests

### Existing tests updated
- lazy-test.janet, phase7-test.janet: updated for nil-safe core functions

## Result
47 test files, all passing. 19 new functions/macros implemented.
Clojure atom semantics complete: validators, watches, metadata, CAS, swap-vals!/reset-vals!.

test/clojure-for-test.janet

@@ -0,0 +1,35 @@
+# Ported from clojure/test_clojure/for.clj
+(use ../src/jolt/api)
+(defn ct-eval [ctx s] (eval-string ctx s))
+
+(print "Ported For Tests")
+
+# --- When ---
+(print "test-for :when...")
+(let [ctx (init)]
+  (assert (= true (ct-eval ctx
+    "(= (for [x (range 10) :when (odd? x)] x)
+        (quote (1 3 5 7 9)))")) "for :when odd?")
+  (assert (= true (ct-eval ctx
+    "(= (for [x (range 4) y (range 4) :when (odd? y)] [x y])
+        (quote ([0 1] [0 3] [1 1] [1 3] [2 1] [2 3] [3 1] [3 3])))")) "for nested :when"))
+(print "  ok")
+
+# --- Let ---
+(print "test-for :let...")
+(let [ctx (init)]
+  (assert (= true (ct-eval ctx
+    "(= (for [x (range 3) y (range 3) :let [z (+ x y)] :when (odd? z)] [x y z])
+        (quote ([0 1 1] [1 0 1] [1 2 3] [2 1 3])))")) "for :let :when"))
+(print "  ok")
+
+# --- Nesting ---
+(print "test-for nesting...")
+(let [ctx (init)]
+  (assert (= true (ct-eval ctx
+    "(= (for [x (quote (a b)) y (interpose x (quote (1 2))) z (list x y)] [x y z])
+        (quote ([a 1 a] [a 1 1] [a a a] [a a a] [a 2 a] [a 2 2]
+                [b 1 b] [b 1 1] [b b b] [b b b] [b 2 b] [b 2 2])))")) "for nested interpose"))
+(print "  ok")
+
+(print "\nAll Ported For tests passed!")