Add common, media_ctrl and v4l2_ctrl modules with CLI drivers and docs

Modules (src/modules/):
- common/error: App_Error struct with structured detail union, error codes,
  app_error_print(); designed for future upgrade to preprocessor-generated
  location codes
- media_ctrl: media device enumeration, topology query (entities/pads/links),
  link enable/disable via Media Controller API (/dev/media*)
- v4l2_ctrl: control enumeration (with menu item fetching), get/set via
  V4L2 ext controls API, device discovery (/dev/video*)

All modules use -std=c11 -D_GNU_SOURCE, build artifacts go to build/ only.
Kernel-version-dependent constants guarded with #ifdef + #warning.

CLI drivers (dev/cli/):
- media_ctrl_cli: list, info, topology, set-link subcommands
- v4l2_ctrl_cli: list, controls, get, set subcommands

Docs (docs/cli/):
- media_ctrl_cli.md and v4l2_ctrl_cli.md with usage, examples, and
  context within the video routing system

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

conventions.md

@@ -2,7 +2,8 @@
 
 ## Language
 
-- **C11** throughout
+- **C11** throughout (`-std=c11`)
+- **`_GNU_SOURCE`** defined via `-D_GNU_SOURCE` in all Makefiles — enables the full GNU/Linux feature set (POSIX 2008, BSD, GNU and Linux-specific extensions); do not define this in source files
 - Target platform: Linux (V4L2, epoll, etc. are Linux-specific)
 
 ---