Add SX tree tools: comprehension, editing, and MCP server

Phase 1-3 of the SX Tools plan — structural reading, editing, and
MCP server for .sx files.

lib/tree-tools.sx — Pure SX functions for tree comprehension and editing:
  Comprehension: annotate-tree, summarise, read-subtree, get-context,
    find-all, get-siblings, validate, navigate
  Editing: replace-node, insert-child, delete-node, wrap-node, tree-set
  Helpers: list-replace, list-insert, list-remove, replace-placeholder

lib/tests/test-tree-tools.sx — 107 tests covering all functions.

hosts/ocaml/bin/mcp_tree.ml — MCP server (stdio JSON-RPC) exposing
  11 tools. Loads tree-tools.sx into the OCaml evaluator, parses .sx
  files with the native parser, calls SX functions for tree operations.

The MCP server can be configured in Claude Code's settings.json as:
  "mcpServers": { "sx-tree": { "command": "path/to/mcp_tree.exe" } }

1429 tests passing (1322 existing + 107 new tree-tools).

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

hosts/javascript/run_tests.js

@@ -282,7 +282,7 @@ for (const expr of frameworkExprs) {
 // Load compiler + VM from lib/ when running full tests
 if (fullBuild) {
   const libDir = path.join(projectDir, "lib");
-  for (const libFile of ["bytecode.sx", "compiler.sx", "vm.sx"]) {
+  for (const libFile of ["bytecode.sx", "compiler.sx", "vm.sx", "tree-tools.sx"]) {
     const libPath = path.join(libDir, libFile);
     if (fs.existsSync(libPath)) {
       const src = fs.readFileSync(libPath, "utf8");