Route yarn block and stanza BOMs through split_bom

#1057 added formats/yarn/blocks.rs and stanzas.rs with their own
leading-BOM splits, which the new guard rejects once main is merged.
Both now call formats::text::split_bom, whose BOM half is now
&'static str (it is always "" or U+FEFF) so callers can store it.

No behavior change.

Assisted-by: Claude Code:claude-opus-5-5

Author: claude

crates/socket-patch-core/src/formats/text.rs

@@ -12,7 +12,7 @@
 
 /// `(bom, rest)`: a leading UTF-8 BOM split off (`""` when there is none),
 /// so an edit can read `rest` and restore `bom` byte-exact on write.
-pub fn split_bom(text: &str) -> (&str, &str) {
+pub fn split_bom(text: &str) -> (&'static str, &str) {
     match text.strip_prefix('\u{feff}') {
         Some(rest) => ("\u{feff}", rest),
         None => ("", text),

crates/socket-patch-core/src/formats/yarn/blocks.rs

@@ -7,6 +7,7 @@
 //! apart on what a block, a key or a field is.
 
 use super::patterns::{berry_npm_alias_target, split_berry_key_patterns, split_pattern};
+use crate::formats::text::split_bom;
 use crate::vendor::common::detect_eol;
 
 /// One key-line block of a yarn lockfile (classic or berry).
@@ -42,10 +43,9 @@ pub(crate) fn scan_blocks(text: &str) -> Vec<LockBlock> {
         }
         let mut content = content.strip_suffix('\r').unwrap_or(content);
         if start == 0 {
-            if let Some(rest) = content.strip_prefix('\u{feff}') {
-                start = '\u{feff}'.len_utf8();
-                content = rest;
-            }
+            let (bom, rest) = split_bom(content);
+            start = bom.len();
+            content = rest;
         }
         lines.push((start, pos, content, terminated));
     }

crates/socket-patch-core/src/formats/yarn/stanzas.rs

@@ -13,6 +13,7 @@
 
 use std::borrow::Cow;
 
+use crate::formats::text::split_bom;
 use crate::utils::line_endings::{to_lf, LineEndings};
 
 /// A berry lock split into its stanzas (see the module docs).
@@ -31,10 +32,7 @@ impl BerryStanzas {
     /// Split `raw`. A [`LineEndings::Mixed`] lock must be refused by the
     /// caller first; one would be rendered back in a single style.
     pub(crate) fn parse(raw: &str) -> Self {
-        let (bom, body) = match raw.strip_prefix('\u{feff}') {
-            Some(rest) => ("\u{feff}", rest),
-            None => ("", raw),
-        };
+        let (bom, body) = split_bom(raw);
         let eol = LineEndings::of(body);
         let lf = to_lf(body).into_owned();
         let trimmed = lf.trim_end_matches('\n');