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>

src/modules/common/Makefile

@@ -0,0 +1,17 @@
+ROOT   := $(abspath ../../..)
+CC      = gcc
+CFLAGS  = -std=c11 -Wall -Wextra -D_GNU_SOURCE -I$(ROOT)/include
+BUILD   = $(ROOT)/build/common
+
+.PHONY: all clean
+
+all: $(BUILD)/error.o
+
+$(BUILD)/error.o: error.c $(ROOT)/include/error.h | $(BUILD)
+	$(CC) $(CFLAGS) -c -o $@ $<
+
+$(BUILD):
+	mkdir -p $@
+
+clean:
+	rm -f $(BUILD)/error.o