reorg: rename library/runtime -> library/kernel (groundwork)

First step of splitting the runtime dumping ground. Pure directory rename
(git mv library/runtime library/kernel) + the six build.zig path references
repointed. The module is still named "runtime" for now; the next commits split
it into concern modules (ipc, memory, process, time, logging, file-system, ...),
dissolve system.zig, and delete the runtime aggregator.

The library/kernel name follows the kernel32 model: it is the userspace library
that wraps the private kernel ABI, distinct from system/kernel/ (the kernel).

zig build green.

library/kernel/block.zig

@@ -0,0 +1,79 @@
+//! Block-device client: the helper a filesystem uses to read and write a block
+//! device (a USB stick, via usb-storage) without hand-rolling the block-protocol
+//! IPC. Layered over `ipc` and the shared `block-protocol` wire format, like
+//! `runtime.usb` over the transfer protocol.
+//!
+//! Transfers name a caller-owned DMA buffer by physical address (from
+//! `runtime.dma.alloc`), so whole sectors move without crossing the IPC size
+//! limit — the same handoff usb-storage uses toward the controller.
+
+const std = @import("std");
+const ipc = @import("ipc.zig");
+const system = @import("system.zig");
+const block_protocol = @import("block-protocol");
+
+pub const Geometry = struct { block_size: u32, block_count: u64 };
+
+pub const Device = struct {
+    endpoint: ipc.Handle,
+
+    /// The device's block size and total block count.
+    pub fn geometry(self: Device) ?Geometry {
+        var request = block_protocol.Request{ .operation = @intFromEnum(block_protocol.Operation.geometry), .lba = 0, .count = 0, .physical = 0 };
+        var reply: [block_protocol.reply_size]u8 = undefined;
+        const n = ipc.call(self.endpoint, std.mem.asBytes(&request), &reply) catch return null;
+        if (n < block_protocol.reply_size) return null;
+        const result = std.mem.bytesToValue(block_protocol.Reply, reply[0..block_protocol.reply_size]);
+        if (result.status != 0) return null;
+        return .{ .block_size = result.block_size, .block_count = result.block_count };
+    }
+
+    /// Read `count` blocks starting at `lba` into the DMA buffer at `physical`.
+    pub fn read(self: Device, lba: u64, count: u32, physical: u64) bool {
+        return self.transfer(.read, lba, count, physical);
+    }
+
+    /// Write `count` blocks starting at `lba` from the DMA buffer at `physical`.
+    pub fn write(self: Device, lba: u64, count: u32, physical: u64) bool {
+        return self.transfer(.write, lba, count, physical);
+    }
+
+    /// Commit any device write cache to stable media (SCSI SYNCHRONIZE CACHE), so
+    /// prior writes survive a power-off. A filesystem calls this before the machine
+    /// goes down; no data transfer, so the buffer arguments are unused.
+    pub fn flush(self: Device) bool {
+        return self.transfer(.flush, 0, 0, 0);
+    }
+
+    fn transfer(self: Device, operation: block_protocol.Operation, lba: u64, count: u32, physical: u64) bool {
+        var request = block_protocol.Request{ .operation = @intFromEnum(operation), .lba = lba, .count = count, .physical = physical };
+        var reply: [block_protocol.reply_size]u8 = undefined;
+        const n = ipc.call(self.endpoint, std.mem.asBytes(&request), &reply) catch return false;
+        if (n < block_protocol.reply_size) return false;
+        return std.mem.bytesToValue(block_protocol.Reply, reply[0..block_protocol.reply_size]).status == 0;
+    }
+};
+
+/// One lookup attempt, no waiting — for a server that retries on its own
+/// timer (the fat service) instead of blocking its harness in here.
+pub fn tryOpen() ?Device {
+    if (ipc.lookup(.block)) |handle| return .{ .endpoint = handle };
+    return null;
+}
+
+/// Look up the block device, retrying generously while the USB storage chain
+/// (controller reset, enumeration, mass-storage bring-up) comes up.
+pub fn open() ?Device {
+    // Patient: the whole USB storage chain (firmware discovery, xHCI reset and
+    // enumeration, mass-storage bring-up) must complete first, which can take
+    // tens of seconds under emulation.
+    var attempts: usize = 0;
+    // 30 s covers the slowest observed healthy chain (a flaky QEMU enumeration
+    // completed at ~24 s); a machine whose stick genuinely failed setup should
+    // not sit a further minute pretending otherwise.
+    while (attempts < 600) : (attempts += 1) {
+        if (ipc.lookup(.block)) |handle| return .{ .endpoint = handle };
+        system.sleep(50);
+    }
+    return null;
+}