Add TCO to evaluator, update SX docs messaging

Evaluator: add _Thunk + _trampoline for tail-call optimization in
lambdas, components, if/when/cond/case/let/begin. All callers in
html.py, resolver.py, handlers.py, pages.py, jinja_bridge.py, and
query_registry.py unwrap thunks at non-tail positions.

SX docs: update tagline to "s-expressions for the web", rewrite intro
to reflect that SX replaces most JavaScript need, fix "What sx is not"
to acknowledge macros and TCO exist.

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

shared/sx/query_registry.py

@@ -78,7 +78,8 @@ def clear(service: str | None = None) -> None:
 def load_query_file(filepath: str, service_name: str) -> list[QueryDef]:
     """Parse an .sx file and register any defquery definitions."""
     from .parser import parse_all
-    from .evaluator import _eval
+    from .evaluator import _eval as _raw_eval, _trampoline
+    _eval = lambda expr, env: _trampoline(_raw_eval(expr, env))
     from .jinja_bridge import get_component_env
 
     with open(filepath, encoding="utf-8") as f:
@@ -102,7 +103,8 @@ def load_query_file(filepath: str, service_name: str) -> list[QueryDef]:
 def load_action_file(filepath: str, service_name: str) -> list[ActionDef]:
     """Parse an .sx file and register any defaction definitions."""
     from .parser import parse_all
-    from .evaluator import _eval
+    from .evaluator import _eval as _raw_eval, _trampoline
+    _eval = lambda expr, env: _trampoline(_raw_eval(expr, env))
     from .jinja_bridge import get_component_env
 
     with open(filepath, encoding="utf-8") as f: