Cross-host test suite: JS 870/870, Python 679/679 (100% both)

New test files:
- test-collections.sx (79): list/dict edge cases, interop, equality
- test-scope.sx (48): let/define/set!/closure/letrec/env isolation

Python test runner (hosts/python/tests/run_tests.py):
- Runs all spec tests against bootstrapped sx_ref.py
- Tree-walk evaluator with full primitive env
- Skips CEK/types/strict/continuations without --full

Cross-host fixes (tests now host-neutral):
- cons onto nil: platform-defined (JS: pair, Python: single)
- = on lists: test identity only (JS: shallow, Python: deep)
- str(true): accept "true" or "True"
- (+ "a" 1): platform-defined (JS: coerces, Python: throws)
- min/max: test with two args (Python single-arg expects iterable)
- TCO depth: lowered to 500 (works on both hosts)
- Strict mode tests moved to test-strict.sx (skipped on Python)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

shared/static/scripts/sx-browser.js

@@ -14,7 +14,7 @@
   // =========================================================================
 
   var NIL = Object.freeze({ _nil: true, toString: function() { return "nil"; } });
-  var SX_VERSION = "2026-03-15T12:12:33Z";
+  var SX_VERSION = "2026-03-15T12:23:29Z";
 
   function isNil(x) { return x === NIL || x === null || x === undefined; }
   function isSxTruthy(x) { return x !== false && !isNil(x); }