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/resolver.py

@@ -31,7 +31,11 @@ import asyncio
 from typing import Any
 
 from .types import Component, Keyword, Lambda, NIL, Symbol
-from .evaluator import _eval
+from .evaluator import _eval as _raw_eval, _trampoline
+
+def _eval(expr, env):
+    """Evaluate and unwrap thunks — all resolver.py _eval calls are non-tail."""
+    return _trampoline(_raw_eval(expr, env))
 from .html import render as html_render, _RawHTML
 from .primitives_io import (
     IO_PRIMITIVES,