Add BLAKE3 hash to GRUB source, remove redundant chain module

Add blake3 checksum for grub-2.12.tar.xz for recipe integrity
verification. Remove the chain module from grub-mkimage — the
chainloader command is built-in in GRUB 2.12, no separate module
needed. Image shrinks from 540 KiB to 512 KiB. Update module table
and size estimate in GRUB integration plan.

local/docs/GRUB-INTEGRATION-PLAN.md

@@ -75,7 +75,6 @@ The standalone EFI image includes these modules:
 | `search` | Search for files/volumes |
 | `search_fs_uuid` | Search by filesystem UUID |
 | `search_label` | Search by volume label |
-| `chain` | Chainload other bootloaders |
 | `echo` | Print messages |
 | `test` | Conditional expressions |
 | `ls` | List files and devices |
@@ -83,6 +82,8 @@ The standalone EFI image includes these modules:
 | `halt` | Shut down |
 | `reboot` | Reboot |
 
+Note: `chainloader` is a built-in command in GRUB 2.12 (no separate module needed).
+
 No RedoxFS module is needed — GRUB chainloads the Redox bootloader instead.
 
 ## GRUB Configuration
@@ -114,9 +115,9 @@ mode with different kernel parameters, once supported).
 
 ## ESP Size Requirements
 
-| Component | Measured Size |
+| Component | Typical Size |
 |-----------|--------------|
-| GRUB EFI binary (with modules) | 540 KiB |
+| GRUB EFI binary (with modules) | ~500 KiB (varies with module list) |
 | Redox bootloader | 100–200 KiB |
 | grub.cfg | < 1 KiB |
 | **Total** | **~1 MiB** |