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>

shared/sx/ocaml_bridge.py

@@ -434,13 +434,13 @@ class OcamlBridge:
                         skipped += 1
                         _logger.warning("OCaml load skipped %s: %s",
                                         filepath, e)
-                # Compile adapter to VM after all files loaded (inside lock)
+                # Register JIT hook — lambdas compile on first call
                 try:
                     await self._send('(vm-compile-adapter)')
                     await self._read_until_ok(ctx=None)
-                    _logger.info("VM adapter compiled — aser runs on bytecode VM")
+                    _logger.info("JIT hook registered — lambdas compile on first call")
                 except OcamlBridgeError as e:
-                    _logger.warning("VM adapter compilation skipped: %s", e)
+                    _logger.warning("JIT hook registration skipped: %s", e)
             _logger.info("Loaded %d definitions from .sx files into OCaml kernel (%d skipped)",
                          count, skipped)
         except Exception as e: