Fix hyperscript conformance: 54/112 passing (was 31/81 baseline)

Runtime visibility fix:
- eval-hs now injects runtime helpers (hs-add, hs-falsy?, hs-strict-eq,
  hs-type-check, hs-matches?, hs-contains?, hs-coerce) via outer let
  binding so the tree-walker evaluator can resolve them

Parser fixes:
- null/undefined: return (null-literal) AST node instead of bare nil
  (nil was indistinguishable from "no parse result" sentinel)
- === / !== tokenized as single 3-char operators
- mod operator: emit (modulo) instead of (%) — modulo is a real primitive

Compiler fixes:
- null-literal → nil
- % → modulo
- contains? → hs-contains? (avoids tree-walker primitive arity conflict)

Runtime additions:
- hs-contains?: wraps list membership + string containment

Tokenizer:
- Added keywords: a, an (removed — broke all tokenization), exist
- Triple operators: === and !== now tokenized correctly

Scorecard: 54/112 test groups passing, +23 from baseline.
Unlocked: really-equals, english comparisons, is-in, null is empty,
null exists, type checks, strict equality, mod.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/hyperscript/runtime.sx

@@ -172,13 +172,7 @@
     (n thunk)
     (define
       do-repeat
-      (fn
-        (i)
-        (when
-          (< i n)
-          (log (str "[hs-repeat] iteration " i " of " n))
-          (thunk)
-          (do-repeat (+ i 1)))))
+      (fn (i) (when (< i n) (thunk) (do-repeat (+ i 1)))))
     (do-repeat 0)))
 
 ;; Repeat forever (until break — relies on exception/continuation).
@@ -310,4 +304,13 @@
     (if
       (string? target)
       (if (= pattern ".*") true (string-contains? target pattern))
-      false)))
+      false)))
+
+(define
+  hs-contains?
+  (fn
+    (collection item)
+    (cond
+      ((list? collection) (some (fn (x) (= x item)) collection))
+      ((string? collection) (string-contains? collection item))
+      (true false))))