Make test.sx self-executing: evaluators run it directly, no codegen

test.sx now defines deftest/defsuite as macros. Any host that provides
5 platform functions (try-call, report-pass, report-fail, push-suite,
pop-suite) can evaluate the file directly — no bootstrap compilation
step needed for JS.

- Added defmacro for deftest (wraps body in thunk, catches via try-call)
- Added defmacro for defsuite (push/pop suite context stack)
- Created run.js: sx-browser.js evaluates test.sx directly (81/81 pass)
- Created run.py: Python evaluator evaluates test.sx directly (81/81 pass)
- Deleted bootstrap_test_js.py and generated test_sx_spec.js
- Updated testing docs page to reflect self-executing architecture

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

shared/sx/tests/test_sx_spec.py

@@ -18,7 +18,7 @@ _PREAMBLE = '''(define assert-equal (fn (expected actual) (assert (equal? expect
 (define assert-type (fn (expected-type val) (let ((actual-type (if (nil? val) "nil" (if (boolean? val) "boolean" (if (number? val) "number" (if (string? val) "string" (if (list? val) "list" (if (dict? val) "dict" "unknown")))))))) (assert (= expected-type actual-type) (str "Expected type " expected-type " but got " actual-type)))))
 (define assert-length (fn (expected-len col) (assert (= (len col) expected-len) (str "Expected length " expected-len " but got " (len col)))))
 (define assert-contains (fn (item col) (assert (some (fn (x) (equal? x item)) col) (str "Expected collection to contain " (str item)))))
-(define assert-throws (fn (thunk) (platform-assert-throws thunk)))'''
+(define assert-throws (fn (thunk) (let ((result (try-call thunk))) (assert (not (get result "ok")) "Expected an error to be thrown but none was"))))'''
 
 
 def _make_env() -> dict: