refactor(load-graph): make hidden module dependencies explicit

Phase 2 of the load-graph project. I fixed the seven hidden dependencies the classification surfaced, so each module declares what it uses instead of relying on init order.

- system-defaults now requires host-environment and user-constants at runtime. They were eval-when-compile only, but env-bsd-p and user-home-dir are read at load, so the compiled module couldn't load standalone.
- custom-buffer-file, dev-fkeys, calendar-sync, and video-audio-recording require keybindings and drop their (when (boundp 'cj/custom-keymap) ...) shims. The shim silently dropped the C-; binding when the module loaded before keybindings. The explicit require makes the dependency real.
- flycheck-config and mail-config require keybindings for their cj/custom-keymap bindings (a use-package :map and a direct keymap-set).
- Removed a dead eval-when-compile (defvar cj/custom-keymap) in transcription-config; nothing there used the variable.

No init.el load-order change. keybindings and the foundation modules already load before these, so the requires are no-ops at startup and only fix standalone and test loading.

I verified each fix with a fresh emacs --batch (require 'X), then swept all modules standalone: every one loads or fails only with a clear missing-package message. Full make test, make validate-modules, and an init smoke all pass. Module headers and the inventory's hidden-dependency section are updated to mark the seven resolved.

1 files changed, 4 insertions, 3 deletions

diff --git a/modules/flycheck-config.el b/modules/flycheck-config.el
index 8932a3ac..5626095c 100644
--- a/modules/flycheck-config.el
+++ b/modules/flycheck-config.el
@@ -10,9 +10,8 @@
 ;;   candidate.
 ;; Top-level side effects: package configuration via use-package, binds into
 ;;   cj/custom-keymap through use-package :map.
-;; Runtime requires: keybindings is needed for the cj/custom-keymap :map binding
-;;   but is not required here; the binding fails standalone. Phase 2 fix.
-;; Direct test load: conditional (needs cj/custom-keymap for the :map binding).
+;; Runtime requires: keybindings.
+;; Direct test load: yes (requires keybindings explicitly).
 ;;
 ;; This file configures Flycheck for on-demand syntax and grammar checking.
 ;; - Flycheck starts automatically only in sh-mode and emacs-lisp-mode
@@ -44,6 +43,8 @@
 
 ;;; Code:
 
+(require 'keybindings) ;; provides cj/custom-keymap (use-package :map below)
+
 (defun cj/prose-helpers-on ()
   "Ensure that `abbrev-mode' and `flycheck-mode' are on in the current buffer."
   (interactive)