Add self-hosting SX test spec: 81 tests bootstrap to Python + JS

The test framework is written in SX and tests SX — the language proves
its own correctness. test.sx defines assertion helpers (assert-equal,
assert-true, assert-type, etc.) and 15 test suites covering literals,
arithmetic, comparison, strings, lists, dicts, predicates, special forms,
lambdas, higher-order forms, components, macros, threading, truthiness,
and edge cases.

Two bootstrap compilers emit native tests from the same spec:
- bootstrap_test.py → pytest (81/81 pass)
- bootstrap_test_js.py → Node.js TAP using sx-browser.js (81/81 pass)

Also adds missing primitives to spec and Python evaluator: boolean?,
string-length, substring, string-contains?, upcase, downcase, reverse,
flatten, has-key?. Fixes number? to exclude booleans, append to
concatenate lists.

Includes testing docs page in SX app at /specs/testing.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/primitives.sx

@@ -208,10 +208,16 @@
   :returns "boolean"
   :doc "True if x is nil/null/None.")
 
+(define-primitive "boolean?"
+  :params (x)
+  :returns "boolean"
+  :doc "True if x is a boolean (true or false). Must be checked before
+        number? on platforms where booleans are numeric subtypes.")
+
 (define-primitive "number?"
   :params (x)
   :returns "boolean"
-  :doc "True if x is a number (int or float).")
+  :doc "True if x is a number (int or float). Excludes booleans.")
 
 (define-primitive "string?"
   :params (x)
@@ -277,11 +283,36 @@
   :returns "string"
   :doc "Uppercase string.")
 
+(define-primitive "upcase"
+  :params (s)
+  :returns "string"
+  :doc "Alias for upper. Uppercase string.")
+
 (define-primitive "lower"
   :params (s)
   :returns "string"
   :doc "Lowercase string.")
 
+(define-primitive "downcase"
+  :params (s)
+  :returns "string"
+  :doc "Alias for lower. Lowercase string.")
+
+(define-primitive "string-length"
+  :params (s)
+  :returns "number"
+  :doc "Length of string in characters.")
+
+(define-primitive "substring"
+  :params (s start end)
+  :returns "string"
+  :doc "Extract substring from start (inclusive) to end (exclusive).")
+
+(define-primitive "string-contains?"
+  :params (s needle)
+  :returns "boolean"
+  :doc "True if string s contains substring needle.")
+
 (define-primitive "trim"
   :params (s)
   :returns "string"
@@ -382,13 +413,23 @@
 (define-primitive "append"
   :params (coll x)
   :returns "list"
-  :doc "Append x to end of coll (returns new list).")
+  :doc "If x is a list, concatenate. Otherwise append x as single element.")
 
 (define-primitive "append!"
   :params (coll x)
   :returns "list"
   :doc "Mutate coll by appending x in-place. Returns coll.")
 
+(define-primitive "reverse"
+  :params (coll)
+  :returns "list"
+  :doc "Return coll in reverse order.")
+
+(define-primitive "flatten"
+  :params (coll)
+  :returns "list"
+  :doc "Flatten one level of nesting. Nested lists become top-level elements.")
+
 (define-primitive "chunk-every"
   :params (coll n)
   :returns "list"
@@ -421,6 +462,11 @@
   :returns "dict"
   :doc "Merge dicts left to right. Later keys win. Skips nil.")
 
+(define-primitive "has-key?"
+  :params (d key)
+  :returns "boolean"
+  :doc "True if dict d contains key.")
+
 (define-primitive "assoc"
   :params (d &rest pairs)
   :returns "dict"