HS: cookie API (+3 tests, partial)

Three-part change: (a) tests/hs-run-filtered.js gets a per-test
__hsCookieStore Map, a globalThis.cookies Proxy, and a
document.cookie getter/setter that reads/writes the store. Per-test
reset clears the store. (b) generate-sx-tests.py declares cookies in
the test header and emits hand-rolled deftests for basic set / update
/ length-when-empty (the three tractable tests). (c) regenerated
spec/tests/test-hyperscript-behavioral.sx via mcp_hs_test.regen.

No .sx edits — `set cookies.foo to 'bar'` already compiles to
(dom-set-prop cookies "foo" "bar") which routes through host-set!.

Suite hs-upstream-expressions/cookies: 0/5 → 3/5.
Smoke 0-195 unchanged at 170/195.

Remaining `basic clear` (needs hs-method-call host-call dispatch) and
`iterate` (needs hs-for-each host-array recognition) need runtime.sx
edits — deferred to a future sx-tree worktree.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/playwright/generate-sx-tests.py

@@ -1832,6 +1832,36 @@ def generate_eval_only_test(test, idx):
     lines = []
     safe_name = sx_name(test['name'])
 
+    # Special case: cluster-33 cookie tests. Each test calls a sequence of
+    # `_hyperscript("HS")` inside `page.evaluate(()=>{...})`. The runner backs
+    # `cookies` with a Proxy over a per-test `__hsCookieStore` map (see
+    # tests/hs-run-filtered.js). Tests handled: basic set, length-when-empty,
+    # update. clear/iterate stay SKIP (need hs-method-call→host-call dispatch
+    # and host-array iteration in hs-for-each — out of cluster-33 scope).
+    if test['name'] == 'basic set cookie values work':
+        return (
+            f'  (deftest "{safe_name}"\n'
+            f'    (hs-cleanup!)\n'
+            f'    (assert (nil? (eval-hs "cookies.foo")))\n'
+            f'    (eval-hs "set cookies.foo to \'bar\'")\n'
+            f'    (assert= (eval-hs "cookies.foo") "bar"))'
+        )
+    if test['name'] == 'update cookie values work':
+        return (
+            f'  (deftest "{safe_name}"\n'
+            f'    (hs-cleanup!)\n'
+            f'    (eval-hs "set cookies.foo to \'bar\'")\n'
+            f'    (assert= (eval-hs "cookies.foo") "bar")\n'
+            f'    (eval-hs "set cookies.foo to \'doh\'")\n'
+            f'    (assert= (eval-hs "cookies.foo") "doh"))'
+        )
+    if test['name'] == 'length is 0 when no cookies are set':
+        return (
+            f'  (deftest "{safe_name}"\n'
+            f'    (hs-cleanup!)\n'
+            f'    (assert= (eval-hs "cookies.length") 0))'
+        )
+
     # Special case: logAll config test. Body sets `_hyperscript.config.logAll = true`,
     # then mutates an element's innerHTML and calls `_hyperscript.processNode`.
     # Our runtime exposes this via hs-set-log-all! + hs-log-captured; we reuse
@@ -2612,6 +2642,7 @@ output.append(';; Bind `window` and `document` as plain SX symbols so HS code th
 output.append(';; references them (e.g. `window.tmp`) can resolve through the host.')
 output.append('(define window (host-global "window"))')
 output.append('(define document (host-global "document"))')
+output.append('(define cookies (host-global "cookies"))')
 output.append('')
 output.append('(define hs-test-el')
 output.append('  (fn (tag hs-src)')