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>

include/v4l2_ctrl.h

@@ -0,0 +1,75 @@
+#pragma once
+
+#include <stdint.h>
+#include "error.h"
+
+/* Opaque handle — defined in v4l2_ctrl.c */
+struct V4l2_Ctrl_Handle;
+
+typedef enum V4l2_Ctrl_Type {
+	CTRL_TYPE_INTEGER      = 1,
+	CTRL_TYPE_BOOLEAN      = 2,
+	CTRL_TYPE_MENU         = 3,
+	CTRL_TYPE_BUTTON       = 4,
+	CTRL_TYPE_INTEGER64    = 5,
+	CTRL_TYPE_CTRL_CLASS   = 6,
+	CTRL_TYPE_STRING       = 7,
+	CTRL_TYPE_BITMASK      = 8,
+	CTRL_TYPE_INTEGER_MENU = 9,
+	CTRL_TYPE_UNKNOWN      = 0xff,
+} V4l2_Ctrl_Type;
+
+struct V4l2_Ctrl_Desc {
+	uint32_t       id;
+	char           name[32];
+	V4l2_Ctrl_Type type;
+	int32_t        min;
+	int32_t        max;
+	int32_t        step;
+	int32_t        default_value;
+	int32_t        current_value;
+	uint32_t       flags;
+};
+
+struct V4l2_Menu_Item {
+	uint32_t index;
+	char     name[32];  /* for CTRL_TYPE_MENU */
+	int64_t  value;     /* for CTRL_TYPE_INTEGER_MENU */
+};
+
+/*
+ * Enumerate all /dev/video* device nodes present on the system.
+ */
+struct App_Error v4l2_ctrl_find_devices(
+	void (*callback)(const char *device_path, void *userdata),
+	void *userdata);
+
+struct App_Error v4l2_ctrl_open(const char *device_path, struct V4l2_Ctrl_Handle **out);
+void            v4l2_ctrl_close(struct V4l2_Ctrl_Handle *handle);
+
+/*
+ * Enumerate all controls on the device.
+ * For menu and integer-menu controls, menu_items is a non-NULL array of
+ * menu_count items. For all other types, menu_items is NULL and menu_count is 0.
+ */
+struct App_Error v4l2_ctrl_enumerate(
+	struct V4l2_Ctrl_Handle *handle,
+	void (*callback)(
+		const struct V4l2_Ctrl_Desc *desc,
+		uint32_t menu_count,
+		const struct V4l2_Menu_Item *menu_items,
+		void *userdata),
+	void *userdata);
+
+struct App_Error v4l2_ctrl_get(
+	struct V4l2_Ctrl_Handle *handle,
+	uint32_t id,
+	int32_t *value_out);
+
+struct App_Error v4l2_ctrl_set(
+	struct V4l2_Ctrl_Handle *handle,
+	uint32_t id,
+	int32_t value);
+
+/* Human-readable name for a control type */
+const char *v4l2_ctrl_type_name(V4l2_Ctrl_Type type);