mind-body: pin the library's public surface as a whole

tests/public_surface.rs reads the library source the way a consumer sees it
and compares every pub item with its signature, pub fields, derives, impl
headers and every path-looking string literal to tests/public_surface.txt.
A new pub static or pub fn under any name, a derive onto Grant, or a root
read from another path fails it. It replaces the three compile_fail
doctests that named with_root, TEST_ROOT and control_root. Dev-dependencies
syn, quote and proc-macro2, already in the graph through serde's derive.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Cargo.lock

@@ -1122,8 +1122,11 @@ dependencies = [
  "eureka-state",
  "huginn-mind",
  "libc",
+ "proc-macro2",
+ "quote",
  "rust_decimal",
  "serde",
+ "syn 2.0.119",
  "tempfile",
 ]