M6: user runtime library rt + C-convention heap

Add lib/ — the shared user-space runtime every user binary links against
(init now, servers/drivers later): syscall wrappers, the heap, IPC stub,
and the process start shim.

- lib/heap.zig: the kernel first-fit free-list ported to user space, grown
  via the mmap syscall instead of pmm+mapPage. Dual API over one global free
  list: extern "C" malloc/free/calloc/realloc (C ABI for future C code) and a
  std.mem.Allocator adapter (with in-place resize) for Zig std containers.
- lib/syscall.zig + sys.zig: raw syscall0..5 (arg3 in r10) and typed
  yield/write/sleep/exit/mmap/munmap over danos.Syscall.
- lib/start.zig: naked _start -> rt_start -> root.main() (SysV realign via
  call), panic -> exit(127).
- lib/user.ld: the user link script, moved from sbin/linker.ld (shared by all
  user binaries).
- build.zig: register the `rt` module; add an addUserBinary() helper that is
  the one recipe for every user binary (freestanding, .large, use_lld,
  user.ld, image_base), replacing the bespoke init block.
- sbin/init.zig: migrated onto rt; drops its hand-rolled syscall2/shims. Now
  proves the heap (alloc -> write from a heap pointer -> free) before the
  heartbeat loop. Serial shows "init: heap ok". Suite 28/28.

lib/sys.zig

@@ -0,0 +1,52 @@
+//! Typed syscall surface for user space — thin wrappers over the raw `syscall`
+//! stubs, one per kernel call. Numbers come from `danos.Syscall`, the single
+//! source of truth shared with the kernel dispatcher.
+
+const danos = @import("danos");
+const sc = @import("syscall.zig");
+
+/// `mmap` protection flags (matching the usual C bit values). Grants are always
+/// readable+writable today; the kernel does not yet honour finer prot.
+pub const PROT_READ: usize = danos.prot_read;
+pub const PROT_WRITE: usize = danos.prot_write;
+pub const PROT_EXEC: usize = danos.prot_exec;
+
+/// Give up the rest of this quantum.
+pub fn yield() void {
+    _ = sc.syscall0(.yield);
+}
+
+/// Write raw bytes to the kernel log (a bring-up diagnostic; real output goes
+/// through the console/VFS later). Returns the byte count, or a wrapped -1.
+pub fn write(msg: []const u8) usize {
+    return sc.syscall2(.debug_write, @intFromPtr(msg.ptr), msg.len);
+}
+
+/// Block the caller for `ms` milliseconds.
+pub fn sleep(ms: usize) void {
+    _ = sc.syscall1(.sleep, ms);
+}
+
+/// End the process. Never returns.
+pub fn exit(code: usize) noreturn {
+    _ = sc.syscall1(.exit, code);
+    unreachable; // the kernel never returns from exit
+}
+
+/// Grant `len` bytes (rounded up to whole pages) of fresh, zeroed, writable
+/// memory and return the base virtual address. On failure returns a value in the
+/// top page (see `mmapFailed`). The user heap grows through this call.
+pub fn mmap(len: usize, prot: usize) usize {
+    return sc.syscall2(.mmap, len, prot);
+}
+
+/// Release a range previously handed out by `mmap`.
+pub fn munmap(base: usize, len: usize) usize {
+    return sc.syscall2(.munmap, base, len);
+}
+
+/// Whether an `mmap` return value is an error (the kernel returns a wrapped
+/// -errno, which lands in the top page — no real grant base is ever that high).
+pub inline fn mmapFailed(ret: usize) bool {
+    return ret > ~@as(usize, 0) - 4095;
+}