home/develop/emacs: add yasnippet snippets and make snippet dirs configurable

This commit makes two significant changes to the Emacs configuration:

1. Refactors emacs.nix to use proper NixOS module structure:
   - Converts from simple configuration to a module with options/config
   - Adds `tb.emacs.extraSnippetDirs` option to allow per-machine snippet
     directories (defaults to ~/.emacs.d/snippets/)
   - Moves yasnippet configuration from custom.el to nix-generated elisp,
     enabling dynamic snippet directory configuration via Nix

2. Adds a comprehensive collection of yasnippet snippets for multiple modes:
   - C++ mode: 50 snippets for common patterns (class, namespace, operators,
     copy/move semantics, debug helpers, documentation, threading primitives)
   - CC mode (shared C/C++): for, switch, main, include guards, etc.
   - CMake mode: if/else, foreach, function, macro, project boilerplate
   - LaTeX mode: document structure, beamer frames, figures, lists
   - Perl mode: package boilerplate, shebang
   - Python/Shell/Nix/Protobuf/Java/CSS/Dockerfile modes: horizontal rules
     and common patterns
   - Text mode: modeline snippets for locale settings

The snippets use yasnippet's inheritance system via .yas-parents files,
so c++-mode inherits from cc-mode and text-mode, reducing duplication.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

home/develop/emacs/snippets/cc-mode/c

@@ -0,0 +1,6 @@
+# -*- mode: snippet; require-final-newline: nil -*-
+# name: c
+# key: c
+# binding: direct-keybinding
+# --
+/* $1 */

home/develop/emacs/snippets/cc-mode/cc

@@ -0,0 +1,7 @@
+# -*- mode: snippet; require-final-newline: nil -*-
+# name: cc
+# key: cc
+# binding: direct-keybinding
+# --
+/** $1
+ */

home/develop/emacs/snippets/cc-mode/comment-spacerline

@@ -0,0 +1,6 @@
+# -*- mode: snippet; require-final-newline: nil -*-
+# name: comment-spacerline
+# key: ***
+# binding: direct-keybinding
+# --
+/******************************************************************************/

home/develop/emacs/snippets/cc-mode/for

@@ -0,0 +1,8 @@
+# -*- mode: snippet; require-final-newline: nil -*-
+# name: for
+# key: for
+# binding: direct-keybinding
+# --
+for ($1) {
+    $0
+}

home/develop/emacs/snippets/cc-mode/fori

@@ -0,0 +1,7 @@
+# -*- mode: snippet -*-
+# name: fori
+# key: fori
+# --
+for (${1:std::size_t} ${2:i} = ${3:0}; $2 ${4:!= size()}; ++$2) {
+    $0
+}

home/develop/emacs/snippets/cc-mode/guard

@@ -0,0 +1,10 @@
+# -*- mode: snippet -*-
+# name: guard
+# key: guard
+# --
+#ifndef ${1:NAME}_HEADER
+#define $1_HEADER
+
+$0
+
+#endif // $1_HEADER

home/develop/emacs/snippets/cc-mode/in

@@ -0,0 +1,6 @@
+# -*- mode: snippet; require-final-newline: nil -*-
+# name: in
+# key: in
+# binding: direct-keybinding
+# --
+#include <$0>

home/develop/emacs/snippets/cc-mode/inc

@@ -0,0 +1,6 @@
+# -*- mode: snippet; require-final-newline: nil -*-
+# name: inc
+# key: inc
+# binding: direct-keybinding
+# --
+#include "$0"

home/develop/emacs/snippets/cc-mode/main

@@ -0,0 +1,9 @@
+# -*- mode: snippet -*-
+# name: main
+# key: main
+# --
+int main(${1:int argc, char *argv[]})
+{
+    $0    
+    return 0;
+}

home/develop/emacs/snippets/cc-mode/packed

@@ -0,0 +1,5 @@
+# -*- mode: snippet -*-
+# name: packed
+# key: packed
+# --
+__attribute__((__packed__))$0

home/develop/emacs/snippets/cc-mode/switch

@@ -0,0 +1,11 @@
+# -*- mode: snippet -*-
+# name: switch
+# key: case
+# --
+switch (${1:ch}) {
+       case ${2:const}:
+       ${3:a = b};
+       break;
+       ${4:default:
+       ${5:action}}
+}