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/compiler.sx

@@ -266,6 +266,7 @@
           (let
             ((head (first ast)))
             (cond
+              ((= head (quote null-literal)) nil)
               ((= head (quote me)) (quote me))
               ((= head (quote it)) (quote it))
               ((= head (quote event)) (quote event))
@@ -328,7 +329,7 @@
                   (hs-to-sx (nth ast 2))))
               ((= head pct-sym)
                 (list
-                  pct-sym
+                  (quote modulo)
                   (hs-to-sx (nth ast 1))
                   (hs-to-sx (nth ast 2))))
               ((= head (quote empty?))
@@ -342,16 +343,16 @@
                   (quote hs-matches?)
                   (hs-to-sx (nth ast 1))
                   (hs-to-sx (nth ast 2))))
-              ((= head (quote contains?))
+              ((= head (quote hs-contains?))
                 (list
-                  (quote contains?)
+                  (quote hs-contains?)
                   (hs-to-sx (nth ast 1))
                   (hs-to-sx (nth ast 2))))
               ((= head (quote as))
                 (list (quote hs-coerce) (hs-to-sx (nth ast 1)) (nth ast 2)))
               ((= head (quote in?))
                 (list
-                  (quote contains?)
+                  (quote hs-contains?)
                   (hs-to-sx (nth ast 2))
                   (hs-to-sx (nth ast 1))))
               ((= head (quote of))
@@ -575,12 +576,12 @@
                 (list
                   (quote not)
                   (list
-                    (quote contains?)
+                    (quote hs-contains?)
                     (hs-to-sx (nth ast 2))
                     (hs-to-sx (nth ast 1)))))
               ((= head (quote in?))
                 (list
-                  (quote contains?)
+                  (quote hs-contains?)
                   (hs-to-sx (nth ast 2))
                   (hs-to-sx (nth ast 1))))
               ((= head (quote type-check))