Refactor MCP tree server: dispatch table, caching, validation, subprocess cleanup

Break up the 1735-line handle_tool match into 45 individual handler functions
with hashtable-based dispatch. Add mtime-based file parse caching (AST + CST),
consolidated run_command helper replacing 9 bare open_process_in patterns,
require_file/require_dir input validation, and pagination (limit/offset) for
sx_find_across, sx_comp_list, sx_comp_usage. Also includes pending VM changes:
rest-arity support, hyperscript parser, compiler/transpiler updates.

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

spec/evaluator.sx

@@ -4280,7 +4280,7 @@
         (let
           ((jit-result (jit-try-call f args)))
           (cond
-            (nil? jit-result)
+            (jit-skip? jit-result)
             (make-cek-state (lambda-body f) local kont)
             (and (dict? jit-result) (get jit-result "__vm_suspended"))
             (make-cek-suspended