riscv64: integrate minimal flattened devicetree reader and switch timer to time CSR

Punch list §25 item 0.3 NOT complete.

- Added `starkernel/fdt.h` and `fdt.c` for minimal read-only devicetree parsing: sufficient for boot-time lookups such as `timebase-frequency`.
- Bootloader now captures the devicetree blob (DTB) from `EFI_DTB_TABLE_GUID` into `BootInfo::dtb`.
- RISC-V timer subsystem now uses the `time` CSR as the primary timestamp source, abandoning the hardcoded `cycle` frequency assumption.
- Timer rate is read from `timebase-frequency` in the DTB when accessible; otherwise, a fallback value is used with a RELATIVE trust level.
- Integrated the SBI TIME extension for one-shot timer deadlines, ensuring re-arming occurs after each tick to avoid missing heartbeats.

Verified: riscv64 builds clean, boots to the ok> prompt with no regression; `riscv64/timer.c` reports accurate frequencies on QEMU's default firmware.
Signed-off-by: Robert Allan James <robert.allan.james@gmail.com>

include/starkernel/boot_info_offsets.h

@@ -15,19 +15,25 @@
  *  16:  memory_map_descriptor_size  u64  (8)
  *  24:  runtime_services            ptr  (8)
  *  32:  acpi_table                  ptr  (8)
- *  40:  framebuffer                 FramebufferInfo (32)
+ *  40:  dtb                         ptr  (8)
+ *  48:  framebuffer                 FramebufferInfo (32)
  *       .base                       ptr  (8)
  *       .size                       u64  (8)
  *       .width                      u32  (4)
  *       .height                     u32  (4)
  *       .pixels_per_scanline        u32  (4)
  *       .pixel_format               u32  (4)
- *  72:  uefi_boot_services_exited   u8   (1)
- *  73:  [7 bytes padding]
- *  80:  kernel_stack_base           ptr  (8)   ← BOOT_INFO_KERNEL_STACK_BASE_OFFSET
- *  88:  kernel_stack_size           u64  (8)   ← BOOT_INFO_KERNEL_STACK_SIZE_OFFSET
- *  96:  args                        KernelArgs
+ *  80:  uefi_boot_services_exited   u8   (1)
+ *  81:  [7 bytes padding]
+ *  88:  kernel_stack_base           ptr  (8)   ← BOOT_INFO_KERNEL_STACK_BASE_OFFSET
+ *  96:  kernel_stack_size           u64  (8)   ← BOOT_INFO_KERNEL_STACK_SIZE_OFFSET
+ * 104:  args                        KernelArgs
+ *
+ * 2026-08-03, punch-list item 0.3: `dtb` inserted at 40, shifting everything
+ * below it by 8. The _Static_asserts in uefi_loader.c caught the stale
+ * constants immediately — that is what they are for; do not silence them by
+ * moving a field, fix the offsets.
  */
 
-#define BOOT_INFO_KERNEL_STACK_BASE_OFFSET  80
-#define BOOT_INFO_KERNEL_STACK_SIZE_OFFSET  88
+#define BOOT_INFO_KERNEL_STACK_BASE_OFFSET  88
+#define BOOT_INFO_KERNEL_STACK_SIZE_OFFSET  96