Lazy JIT compilation: lambdas compile to bytecode on first call

Replace AOT adapter compilation with lazy JIT — each named lambda is
compiled to VM bytecode on first call, cached in l_compiled field for
subsequent calls. Compilation failures fall back to CEK gracefully.

VM types (vm_code, vm_upvalue_cell, vm_closure) moved to sx_types.ml
mutual recursion block. Lambda and Component records gain mutable
l_compiled/c_compiled cache fields. jit_compile_lambda in sx_vm.ml
wraps body as (fn (params) body), invokes spec/compiler.sx via CEK,
extracts inner closure from OP_CLOSURE constant.

JIT hooks in both paths:
- vm_call: Lambda calls from compiled VM code
- continue_with_call: Lambda calls from CEK step loop (injected by
  bootstrap.py post-processing)

Pre-mark sentinel prevents re-entrancy (compile function itself was
hanging when JIT'd mid-compilation). VM execution errors caught and
fall back to CEK with sentinel marking.

Also: add kbd/samp/var to HTML_TAGS, rebuild sx-browser.js, add page
URL to sx-page-full-py timing log.

Performance: first page 28s (JIT compiles 17 functions), subsequent
pages 0.31s home / 0.71s wittgenstein (was 2.3s). All 1945 tests pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

spec/render.sx

@@ -32,7 +32,7 @@
     "div" "p" "blockquote" "pre" "figure" "figcaption" "address" "details" "summary"
     ;; Inline
     "a" "span" "em" "strong" "small" "b" "i" "u" "s" "mark" "sub" "sup"
-    "abbr" "cite" "code" "time" "br" "wbr" "hr"
+    "abbr" "cite" "code" "kbd" "samp" "var" "time" "br" "wbr" "hr"
     ;; Lists
     "ul" "ol" "li" "dl" "dt" "dd"
     ;; Tables