Rebrand sexp → sx across web platform (173 files)

Rename all sexp directories, files, identifiers, and references to sx.
artdag/ excluded (separate media processing DSL).

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

shared/sx/__init__.py

@@ -0,0 +1,66 @@
+"""
+S-expression language core.
+
+Parse, evaluate, and serialize s-expressions.  This package provides the
+foundation for the composable fragment architecture described in
+``docs/sx-architecture-plan.md``.
+
+Quick start::
+
+    from shared.sx import parse, evaluate, serialize, Symbol, Keyword
+
+    expr = parse('(let ((x 10)) (+ x 1))')
+    result = evaluate(expr)          # → 11
+
+    expr2 = parse('(map (fn (n) (* n n)) (list 1 2 3))')
+    result2 = evaluate(expr2)        # → [1, 4, 9]
+"""
+
+from .types import (
+    NIL,
+    Component,
+    Keyword,
+    Lambda,
+    Symbol,
+)
+from .parser import (
+    ParseError,
+    parse,
+    parse_all,
+    serialize,
+)
+from .evaluator import (
+    EvalError,
+    evaluate,
+    make_env,
+)
+from .primitives import (
+    all_primitives,
+    get_primitive,
+    register_primitive,
+)
+from .env import Env
+
+__all__ = [
+    # Types
+    "Symbol",
+    "Keyword",
+    "Lambda",
+    "Component",
+    "NIL",
+    # Parser
+    "parse",
+    "parse_all",
+    "serialize",
+    "ParseError",
+    # Evaluator
+    "evaluate",
+    "make_env",
+    "EvalError",
+    # Primitives
+    "register_primitive",
+    "get_primitive",
+    "all_primitives",
+    # Environment
+    "Env",
+]