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/error.h

@@ -0,0 +1,59 @@
+#pragma once
+
+#include <errno.h>
+
+typedef enum Error_Code {
+	ERR_NONE      = 0,
+	ERR_SYSCALL   = 1,
+	ERR_INVALID   = 2,
+	ERR_NOT_FOUND = 3,
+} Error_Code;
+
+struct Syscall_Error_Detail {
+	int err_no;
+};
+
+struct Invalid_Error_Detail {
+	/* fields added as concrete cases arise */
+	int placeholder;
+};
+
+struct App_Error {
+	Error_Code  code;
+	const char *file;
+	int         line;
+	union {
+		struct Syscall_Error_Detail syscall;
+		struct Invalid_Error_Detail invalid;
+	} detail;
+};
+
+void app_error_print(struct App_Error *e);
+
+#define APP_OK \
+	((struct App_Error){ .code = ERR_NONE })
+
+#define APP_IS_OK(e) \
+	((e).code == ERR_NONE)
+
+#define APP_SYSCALL_ERROR() \
+	((struct App_Error){ \
+		.code   = ERR_SYSCALL, \
+		.file   = __FILE__, \
+		.line   = __LINE__, \
+		.detail = { .syscall = { .err_no = errno } }, \
+	})
+
+#define APP_INVALID_ERROR() \
+	((struct App_Error){ \
+		.code = ERR_INVALID, \
+		.file = __FILE__, \
+		.line = __LINE__, \
+	})
+
+#define APP_NOT_FOUND_ERROR() \
+	((struct App_Error){ \
+		.code = ERR_NOT_FOUND, \
+		.file = __FILE__, \
+		.line = __LINE__, \
+	})