reorg: move display + scanout protocols to library/protocol

display-protocol -> library/protocol/display/, scanout-protocol ->
library/protocol/scanout/. Consumers import both directly, killing the cryptic
dp/sp aliases in virtio-gpu (now display_protocol / scanout_protocol) and the
runtime.display_protocol / runtime.scanout_protocol re-exports. runtime.display
(the client) still imports display-protocol by name; scanout has no runtime
client, so its runtime module import is dropped too.

zig build + test green; display-service, virtio-gpu, display-native,
display-reattach pass.

library/protocol/scanout/scanout-protocol.zig

@@ -0,0 +1,49 @@
+//! The scanout wire protocol — what the compositor says to a native scanout driver (e.g.
+//! virtio-gpu) over its well-known `.scanout` endpoint to put a composited frame on screen.
+//! The driver owns the panel and the shared scanout surface it handed the compositor (via the
+//! display service's `attach_scanout`); the compositor composites into that surface, then asks
+//! the driver to present a damaged rectangle. Tiny by design — one present request. Separate
+//! from the display protocol because the directions differ: clients call the compositor over
+//! `.display`; the compositor calls the driver over `.scanout`. See docs/display-v2.md.
+
+const std = @import("std");
+
+pub const Operation = enum(u32) {
+    /// present(x, y, width, height): put the given rectangle of the shared scanout surface on
+    /// the panel (on virtio-gpu: transfer-to-host of the region, then a fenced resource flush).
+    present = 0,
+    /// get_modes() -> ModesReply: the display modes this scanout can switch to (V5).
+    get_modes = 1,
+    /// set_mode(width, height): change the scanout resolution — the shared surface is sized to
+    /// the largest mode, so this just re-points the scanout rectangle; the surface is unchanged.
+    set_mode = 2,
+};
+
+pub const Request = extern struct {
+    operation: u32,
+    x: u32 = 0,
+    y: u32 = 0,
+    width: u32 = 0,
+    height: u32 = 0,
+};
+
+pub const Reply = extern struct {
+    status: i32, // 0 on success, negative on failure
+    reserved: u32 = 0,
+};
+
+/// One offered display mode.
+pub const Mode = extern struct { width: u32, height: u32 };
+pub const max_modes = 4;
+
+/// The reply to `get_modes`: a small fixed list of modes.
+pub const ModesReply = extern struct {
+    status: i32,
+    count: u32,
+    modes: [max_modes]Mode,
+};
+
+pub const message_maximum: usize = 64;
+pub const request_size: usize = @sizeOf(Request);
+pub const reply_size: usize = @sizeOf(Reply);
+pub const modes_reply_size: usize = @sizeOf(ModesReply);