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/media_ctrl.h

@@ -0,0 +1,90 @@
+#pragma once
+
+#include <stdint.h>
+#include "error.h"
+
+/* Opaque handle — defined in media_ctrl.c */
+struct Media_Ctrl;
+
+struct Media_Device_Info {
+	char     driver[16];
+	char     model[32];
+	char     serial[40];
+	char     bus_info[32];
+	uint32_t media_version;
+	uint32_t hw_revision;
+	uint32_t driver_version;
+};
+
+struct Media_Entity {
+	uint32_t id;
+	char     name[32];
+	uint32_t type;       /* MEDIA_ENT_F_* value from kernel */
+	uint32_t flags;
+	uint16_t pad_count;
+	uint16_t link_count;
+	uint32_t dev_major;  /* associated device node major, 0 if none */
+	uint32_t dev_minor;
+};
+
+struct Media_Pad {
+	uint32_t entity_id;
+	uint16_t index;
+	uint32_t flags;      /* MEDIA_PAD_FL_SINK / MEDIA_PAD_FL_SOURCE */
+};
+
+struct Media_Link {
+	struct Media_Pad source;
+	struct Media_Pad sink;
+	uint32_t         flags; /* MEDIA_LNK_FL_ENABLED, MEDIA_LNK_FL_IMMUTABLE */
+};
+
+/*
+ * Enumerate all /dev/media* device nodes present on the system.
+ * Calls callback once per device path found.
+ */
+struct App_Error media_ctrl_find_devices(
+	void (*callback)(const char *device_path, void *userdata),
+	void *userdata);
+
+struct App_Error media_ctrl_open(const char *device_path, struct Media_Ctrl **out);
+void            media_ctrl_close(struct Media_Ctrl *ctrl);
+
+struct App_Error media_ctrl_get_info(
+	struct Media_Ctrl *ctrl,
+	struct Media_Device_Info *out);
+
+/*
+ * Enumerate all entities in the media graph.
+ */
+struct App_Error media_ctrl_enum_entities(
+	struct Media_Ctrl *ctrl,
+	void (*callback)(const struct Media_Entity *entity, void *userdata),
+	void *userdata);
+
+/*
+ * Enumerate pads and links for a specific entity.
+ * Pass the entity struct obtained from media_ctrl_enum_entities —
+ * the pad and link counts are taken from it.
+ * Either callback may be NULL if that information is not needed.
+ */
+struct App_Error media_ctrl_enum_entity_pads_and_links(
+	struct Media_Ctrl *ctrl,
+	const struct Media_Entity *entity,
+	void (*pad_callback)(const struct Media_Pad *pad, void *userdata),
+	void (*link_callback)(const struct Media_Link *link, void *userdata),
+	void *userdata);
+
+/*
+ * Enable or disable a link between two pads.
+ * Immutable links cannot be changed and will return an error.
+ */
+struct App_Error media_ctrl_set_link(
+	struct Media_Ctrl *ctrl,
+	uint32_t source_entity_id, uint16_t source_pad_index,
+	uint32_t sink_entity_id,   uint16_t sink_pad_index,
+	int enabled);
+
+/* Human-readable names for display */
+const char *media_entity_type_name(uint32_t type);
+const char *media_pad_flag_name(uint32_t flags);