docs(rust): mark the Unicode case table as frozen and generated

The table is pinned to Unicode 15.0.0 for matching parity and its generator
is no longer in the repository. Say so in the header and mark the file as
generated for linguist.

.gitattributes

@@ -27,6 +27,9 @@ rust_backend/vendor/rustix-1.1.5/ORG_CODE_OF_CONDUCT.md whitespace=-blank-at-eol
 # Preserve libusb's upstream sources, license whitespace, and source hashes.
 rust_backend/vendor/libusb1-sys-0.7.0/** -whitespace
 
+# Frozen Unicode 15.0.0 case table; its generator is no longer in the repository.
+rust_backend/crates/core/src/metadata/musicbrainz/case_data.rs linguist-generated=true
+
 # Binary files
 *.png binary
 *.jpg binary

rust_backend/crates/core/src/metadata/musicbrainz/case_data.rs

@@ -1,5 +1,7 @@
-// Generated by docs/internal/legacy-go/rust_backend/tools/generate_case_data.go. DO NOT EDIT.
-// Unicode 15.0.0, golang.org/x/text v0.41.0; see NOTICE.
+// Generated and frozen. DO NOT EDIT.
+// Pinned to Unicode 15.0.0 (golang.org/x/text v0.41.0; see NOTICE) for matching
+// parity with the retired Go backend. It was generated once by a tool that is no
+// longer in the repository, and must not be regenerated with newer std tables.
 // Flags: cased=1, case-ignorable=2, word break=4, mid punctuation=8.
 #[rustfmt::skip]
 pub(super) const RANGES: &[(u32, u32, u8)] = &[