intel: implement CS submit — Mesa winsys integration (Phase 4 complete)

Implement redox_private_cs_submit() in the Intel GpuDriver,
completing Phase 4 (Render Path). This is the userspace GPU
command submission interface used by Mesa.

- redox_private_cs_submit(): map source GEM buffer to GPU address,
  extract batch commands as u32 slice from src_offset with
  byte_count dwords, submit to render ring via ring.submit_batch()
- Returns RedoxPrivateCsSubmitResult with seqno (0 for now —
  fence integration deferred)

This completes all 4 modules of Phase 4:
  batch.rs ✅  fence.rs ✅  execlists.rs ✅  Mesa winsys ✅

Remaining across all phases:
  Phase 2: DBUF detailed programming
  Phase 3: Atomic modesetting (requires scheme.rs changes)

local/recipes/gpu/redox-drm/source/src/drivers/intel/mod.rs

@@ -32,7 +32,7 @@ use redox_driver_sys::memory::MmioRegion;
 use redox_driver_sys::pci::{PciBarInfo, PciDevice, PciDeviceInfo};
 use redox_driver_sys::quirks::PciQuirkFlags;
 
-use crate::driver::{DriverError, DriverEvent, GpuDriver, Result};
+use crate::driver::{DriverError, DriverEvent, GpuDriver, Result, RedoxPrivateCsSubmit, RedoxPrivateCsSubmitResult};
 use crate::drivers::interrupt::InterruptHandle;
 use crate::gem::{GemHandle, GemManager};
 use crate::kms::connector::{synthetic_edid, Connector};
@@ -758,6 +758,23 @@ impl GpuDriver for IntelDriver {
 
         self.process_irq()
     }
+
+    fn redox_private_cs_submit(
+        &self,
+        submit: &RedoxPrivateCsSubmit,
+    ) -> Result<RedoxPrivateCsSubmitResult> {
+        let src_addr = self.ensure_gem_gpu_mapping(submit.src_handle)?;
+
+        let cmds_ptr = (src_addr + submit.src_offset) as *const u32;
+        let dword_count = (submit.byte_count / 4) as usize;
+        let cmds = unsafe { std::slice::from_raw_parts(cmds_ptr, dword_count) };
+
+        let mut ring = self.ring.lock()
+            .map_err(|_| DriverError::Initialization("Intel ring state poisoned".into()))?;
+        ring.submit_batch(cmds)?;
+
+        Ok(RedoxPrivateCsSubmitResult { seqno: 0 })
+    }
 }
 
 fn detect_display_topology(display: &IntelDisplay, edid_source: Option<&[DpAux]>) -> Result<(Vec<Connector>, Vec<Encoder>)> {