docs(load-graph): seed module inventory and annotate foundation headers

I started the init.el load-graph classification with the foundation batch. I added docs/design/module-inventory.org as the living per-module inventory and annotated the seven foundation modules (system-lib, user-constants, host-environment, system-defaults, keyboard-compat, keybindings, config-utilities) with the load-graph header contract: layer, category, load shape, eager reason, top-level side effects, runtime requires, and direct-test-load safety.

I changed no load order, so init.el keeps its current eager order. The inventory records one hidden dependency for Phase 2: system-defaults uses host-environment and user-constants symbols at load while declaring them eval-when-compile, so the compiled module cannot load standalone.

1 files changed, 10 insertions, 0 deletions

diff --git a/modules/system-lib.el b/modules/system-lib.el
index 801759586..333c15ee2 100644
--- a/modules/system-lib.el
+++ b/modules/system-lib.el
@@ -1,6 +1,16 @@
 ;;; system-lib.el --- System utility library functions -*- lexical-binding: t; -*-
 ;;
 ;;; Commentary:
+;;
+;; Layer: 1 (Foundation).
+;; Category: F/L.
+;; Load shape: eager.
+;; Eager reason: low-level helpers (executable lookup, process output, silent
+;;   logging) used by many eager modules during startup.
+;; Top-level side effects: none.
+;; Runtime requires: none (auth-source loaded on demand inside the helper).
+;; Direct test load: yes (pure helpers; batch-safe).
+;;
 ;; This module provides low-level system utility functions for checking
 ;; the availability of external programs and system capabilities.
 ;;