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, 3 insertions, 4 deletions

diff --git a/modules/mail-config.el b/modules/mail-config.el
index 5050d761..e1ebdfce 100644
--- a/modules/mail-config.el
+++ b/modules/mail-config.el
@@ -10,10 +10,8 @@
 ;;   command-loaded deferral candidate for Phase 5.
 ;; Top-level side effects: registers cj/email-map under cj/custom-keymap, one
 ;;   add-hook, two advice-add, one global key, package config.
-;; Runtime requires: user-constants, system-lib, mu4e-attachments. keybindings
-;;   is needed for the C-; e registration but is not required, so the module
-;;   errors standalone. Phase 2 fix.
-;; Direct test load: conditional (needs cj/custom-keymap from keybindings).
+;; Runtime requires: user-constants, system-lib, mu4e-attachments, keybindings.
+;; Direct test load: yes (requires keybindings; mu4e package must be present).
 ;;
 ;; I found Aime Bertrand's blog post to be an excellent walkthrough of how to
 ;; setup a Mu4e config.
@@ -34,6 +32,7 @@
 (require 'user-constants)
 (require 'system-lib)
 (require 'mu4e-attachments)
+(require 'keybindings)  ;; provides cj/custom-keymap
 
 ;; cj/custom-keymap's real binding is in keybindings.el, which init.el loads
 ;; first. The use-package org-msg :preface below wraps in eval-and-compile, so