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/rt.zig

@@ -0,0 +1,22 @@
+//! danos user-space runtime library — a nascent libc. Every user binary (init,
+//! and later the VFS server + device drivers) imports this as `@import("rt")`:
+//! syscall wrappers, the C-convention heap, IPC helpers, and the process start
+//! shim. It is compiled into each binary (inheriting its `.large` code model and
+//! freestanding target), so all user programs share one implementation.
+//!
+//! A user binary needs three lines:
+//!   const rt = @import("rt");
+//!   pub const panic = rt.panic;
+//!   comptime { _ = &rt.start._start; }   // pull the entry shim in
+//! and a `pub fn main() void`.
+
+pub const sys = @import("sys.zig");
+pub const heap = @import("heap.zig");
+pub const ipc = @import("ipc.zig");
+pub const start = @import("start.zig");
+
+/// Re-exported so a user binary can `pub const panic = rt.panic;`.
+pub const panic = start.panic;
+
+/// The heap as a `std.mem.Allocator`, for Zig `std` containers in user code.
+pub const allocator = heap.allocator;