feat: proper IPC-based boot — init loads servers from storage via IPC, VirtIO GPU driver

Complete microkernel boot pipeline: adapter loads only init + storage + display
from disk. Init bootstraps all other servers by reading binaries from the
storage server via IPC, parsing ELF, creating processes, mapping pages, and
spawning threads.

New syscalls:
- CreateEndpoint: userspace endpoint creation with full capability grant
- DebugPutchar: debug output to UART from userspace
- MapPage: cross-process page mapping (init loads binaries into children)
- SpawnThreadIn: spawn thread in another process via capability

Key fixes:
- Per-process L1 page tables for VPN[2]=0 (prevents cross-process corruption)
- Separate kernel_pt from page_tables[0] (init pid=0 overwrote kernel PT ref)
- Thread Running state set in switch_to_next (syscalls rejected without it)
- Pending IPC results applied during context switch
- SIE disabled during boot (timer can't preempt into unready processes)
- User heap VA starts at 0x40000 (avoids shared buffer at 0x30000)
- RequestPages maps at user VAs via per-process heap allocator

VirtIO GPU:
- Adapter probes for GPU device, initializes resource/scanout/backing
- Display server writes to mapped framebuffer, flushes via GPU virtqueue
- 640x480 BGRA framebuffer with transfer-to-host and resource-flush

Storage server:
- Userspace VirtIO block driver using pre-mapped MMIO and DMA pages
- FAT16 filesystem reader serving file-read requests via IPC
- Shared buffer protocol: init writes filename, storage writes file data

Echo app for IPC testing (adapter-loaded echo proved the pipeline end-to-end).

crates/kernel/src/syscall/ipc.rs

@@ -4,6 +4,31 @@ use super::{SyscallOutcome, SyscallResult};
 use crate::Kernel;
 
 impl<P: Platform> Kernel<P> {
+    pub(crate) fn handle_create_endpoint(
+        &mut self,
+        caller_pid: types::id::ProcessId,
+    ) -> SyscallOutcome {
+        let ep_id = match self.create_endpoint() {
+            Ok(id) => id,
+            Err(e) => return SyscallOutcome::Complete(SyscallResult::Error(e)),
+        };
+
+        let all_perms = types::permission::PermissionSet::empty()
+            .with(types::permission::Permission::Read)
+            .with(types::permission::Permission::Write)
+            .with(types::permission::Permission::Execute)
+            .with(types::permission::Permission::Delegate);
+
+        match self.grant_cap(
+            caller_pid,
+            domain::resource::Resource::Endpoint(ep_id),
+            all_perms,
+        ) {
+            Ok(slot) => SyscallOutcome::Complete(SyscallResult::Slot(slot)),
+            Err(e) => SyscallOutcome::Complete(SyscallResult::Error(e)),
+        }
+    }
+
     pub(crate) fn handle_send(
         &mut self,
         caller_tid: types::id::ThreadId,