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>

spec/tests/test-hyperscript-compiler.sx

@@ -57,12 +57,10 @@
   (deftest
     "addition passes through"
     (let
-      ((sx (hs-to-sx-from-source "1 + 2")))
-      (let
-        ((val (first sx)))
-        (assert= (quote hs-add) (first val))
-        (assert= 1 (nth val 1))
-        (assert= 2 (nth val 2)))))
+      ((val (hs-to-sx-from-source "1 + 2")))
+      (assert= (quote hs-add) (first val))
+      (assert= 1 (nth val 1))
+      (assert= 2 (nth val 2))))
   (deftest
     "comparison emits correctly"
     (let