fix(bios): -D warnings compatibility on nightly rustc

Follow-up to 895c69b: CI's `RUSTFLAGS=-D warnings` promoted four
warnings to errors under nightly — all latent in the BIOS path
until build-images.sh started compiling zamak-stage1/zamak-bios.

- `zamak-stage1/src/mbr.rs`, `zamak-bios/src/entry.rs`: drop the
  `.intel_syntax noprefix` / `.att_syntax prefix` directives that
  wrapped each `global_asm!`. Newer rustc emits `-D bad_asm_style`
  with the message "Intel syntax is the default" — the directives
  were redundant since `global_asm!` on x86 already defaults to
  Intel syntax. The inner asm text is unchanged (still Intel).
- `zamak-bios/src/fat32.rs`: `#[allow(unused_imports)]` on `Vec`
  (used transitively via `vec!`).
- `zamak-bios/src/main.rs`: `#[allow(unused_imports)]` on
  `fat32::Fat32`, `#[allow(dead_code)]` on the VGA-text `println`
  helper kept around for future debug instrumentation.

Verified locally: `RUSTFLAGS='-D warnings' ./zamak-test/build-images.sh`
compiles clean, and both `boot-smoke` (uefi) and `asm-verification`
suites still `[PASS]`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: UnbreakableMJ

zamak-bios/src/entry.rs

@@ -30,8 +30,11 @@ use core::arch::global_asm;
 // sections share labels and data and must be linked as a single contiguous unit.
 // The call_bios_int function alone requires ~40 instructions because it performs
 // a full 32→16→real→16→32 mode round-trip to invoke BIOS interrupts.
+// NOTE: `global_asm!`'s default on x86 is Intel syntax — no
+// `.intel_syntax`/`.att_syntax` directives here (they became
+// `-D bad_asm_style` on modern rustc and CI runs with
+// `-D warnings`).
 global_asm!(
-    ".intel_syntax noprefix",
     // =========================================================================
     // 16-bit Real Mode Entry
     // =========================================================================
@@ -210,5 +213,4 @@ global_asm!(
     ".align 4",
     "esp_save_ptr:",
     "    .long 0",
-    ".att_syntax prefix",
 );

zamak-bios/src/fat32.rs

@@ -3,6 +3,7 @@
 
 use alloc::string::String;
 use alloc::vec;
+#[allow(unused_imports)]
 use alloc::vec::Vec;
 use zamak_core::fs::{BlockDevice, Error, FileEntry, FileSystem, FileType};
 

zamak-bios/src/main.rs

@@ -30,6 +30,7 @@ pub mod vbe;
 
 use core::panic::PanicInfo;
 use disk::Disk;
+#[allow(unused_imports)]
 use fat32::Fat32;
 use mmap::get_memory_map;
 
@@ -461,6 +462,7 @@ pub extern "C" fn kmain(drive_id: u8) -> ! {
     loop {}
 }
 
+#[allow(dead_code)]
 fn println(vga: *mut u8, line: isize, msg: &str, color: u8) {
     for (i, &byte) in msg.as_bytes().iter().enumerate() {
         // SAFETY: vga points to 0xB8000 VGA text buffer (80x25x2 = 4000 bytes);

zamak-stage1/src/mbr.rs

@@ -28,8 +28,11 @@ use core::arch::global_asm;
 // must fit in exactly 512 bytes as a single contiguous block. The boot signature
 // at offset 510 and patchable fields at fixed offsets require .org directives
 // that only work within one assembly section.
+// NOTE: rustc's `global_asm!` default is Intel syntax on x86 targets, so
+// no `.intel_syntax noprefix` / `.att_syntax prefix` directives are
+// emitted around this block — newer nightlies warn about redundant
+// switches and our CI runs with `-D warnings`.
 global_asm!(
-    ".intel_syntax noprefix",
     ".pushsection .mbr, \"ax\"",
     ".code16",
     ".global mbr_start",
@@ -123,5 +126,4 @@ global_asm!(
     // Boot signature at offset 510.
     ".word 0xAA55",
     ".popsection",
-    ".att_syntax prefix",
 );