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/jinja_bridge.py

@@ -169,7 +169,8 @@ def register_components(sx_source: str) -> None:
                   (div :class "..." (div :class "..." title)))))
         ''')
     """
-    from .evaluator import _eval
+    from .evaluator import _eval as _raw_eval, _trampoline
+    _eval = lambda expr, env: _trampoline(_raw_eval(expr, env))
     from .parser import parse_all
     from .css_registry import scan_classes_from_sx