fix(lexer_core): gate unsafe_code expect on x86_64 only

The #[expect(unsafe_code)] on prefetch_buffer() is only fulfilled on
x86_64 where the prefetch intrinsics use unsafe. On aarch64 (macOS CI),
the cfg-gated block is compiled out, leaving the expectation unfulfilled
and triggering -D unfulfilled-lint-expectations.

Use #[cfg_attr(target_arch = "x86_64", expect(...))] so the lint
expectation only exists on the architecture that needs it.

Author: elucidsoft

compiler/ori_lexer_core/src/source_buffer/mod.rs

@@ -212,7 +212,10 @@ fn detect_interior_nulls(source: &[u8], issues: &mut Vec<EncodingIssue>) {
 ///
 /// Warms up L1 cache for the scanner's initial reads. On platforms without
 /// prefetch support, this is a no-op.
-#[expect(unsafe_code, reason = "x86_64 prefetch intrinsics require unsafe")]
+#[cfg_attr(
+    target_arch = "x86_64",
+    expect(unsafe_code, reason = "x86_64 prefetch intrinsics require unsafe")
+)]
 fn prefetch_buffer(buf: &[u8]) {
     #[cfg(target_arch = "x86_64")]
     {