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>

sx/sxc/sx_components.py

@@ -279,14 +279,15 @@ def _docs_introduction_sx() -> str:
         'Components use defcomp with keyword parameters and optional children. '
         'The evaluator supports let bindings, conditionals, lambda, map/filter/reduce, and ~80 primitives.")'
         '    (p :class "text-stone-600"'
-        '      "sx is not trying to replace JavaScript. It\'s trying to replace the pattern of '
+        '      "sx replaces the pattern of '
         'shipping a JS framework + build step + client-side router + state management library '
-        'just to render some server data into HTML."))'
+        'just to render some server data. For most applications, sx eliminates the need for '
+        'JavaScript entirely — htmx attributes handle interactivity, hyperscript handles small behaviours, '
+        'and the server handles everything else."))'
         '  (~doc-section :title "What sx is not" :id "not"'
         '    (ul :class "space-y-2 text-stone-600"'
         '      (li "Not a general-purpose programming language — it\'s a UI rendering language")'
-        '      (li "Not a Lisp implementation — no macros, no continuations, no tail-call optimization")'
-        '      (li "Not a replacement for JavaScript — it handles rendering, not arbitrary DOM manipulation")'
+        '      (li "Not a full Lisp — it has macros and TCO, but no continuations or call/cc")'
         '      (li "Not production-hardened at scale — it runs one website"))))'
     )