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/tokenizer.sx

@@ -477,8 +477,13 @@
                   (< (+ pos 1) src-len)
                   (= (hs-peek 1) "="))
                 (do
-                  (hs-emit! "op" (str ch "=") start)
-                  (hs-advance! 2)
+                  (if
+                    (and
+                      (or (= ch "=") (= ch "!"))
+                      (< (+ pos 2) src-len)
+                      (= (hs-peek 2) "="))
+                    (do (hs-emit! "op" (str ch "==") start) (hs-advance! 3))
+                    (do (hs-emit! "op" (str ch "=") start) (hs-advance! 2)))
                   (scan!))
                 (and
                   (= ch "'")