codegen: propagate bytes_fields to map<K, bytes> values (#76)

[senthil1216]

Summary

• Under use_bytes_type() (or use_bytes_type_in(...)), map<K, bytes> values now become bytes::Bytes instead of Vec<u8> — the lone hold-out among bytes contexts. This lets map values participate in the to_owned_from_source zero-copy slice_ref path that singular / optional / repeated / oneof bytes_fields already use.
• One intentional carve-out: map<bytes, bytes> (only reachable via strict_utf8_mapping(true)) keeps Vec<u8> values because the existing bytes_key_bytes_val_map JSON helper is concrete HashMap<Vec<u8>, Vec<u8>>. Documented on both knobs.
• Generated Arbitrary impls for the affected map fields go through a new __private::arbitrary_bytes_map<K> shim (K: Arbitrary + Eq + Hash — every proto map-key type qualifies). No turbofish needed at the call site.
• Breaking for code already using use_bytes_type() on a proto containing map<K, bytes>: rewrite value construction from Vec<u8> to bytes::Bytes (b"v".to_vec() → bytes::Bytes::from_static(b"v")).

Carve-out logic lives in classify_field::map_value_use_bytes (the source of truth) and is mirrored in impl_message::map_merge_arm, impl_text::map_merge_arm, and view::map_to_owned_expr with parallel "see classify_field" comments.

Closes #76 (item a). Item c (checked_add in bytes_from_source) was already done in #84; item d (string_fields shared-buffer strings) remains for a future PR. Net diff ≈ 120 lines excluding tests.

Test plan

• Updated test_bytes_type_map_value_stays_vec → test_bytes_type_map_value_uses_bytes (positive type assertion plus wire / view→owned / JSON / text agreement).
• New test_bytes_type_map_value_to_owned_from_source_zero_copy asserts slice_ref aliasing into the source Bytes buf for map values — same property the singular/repeated/oneof tests pin.
• Updated test_bytes_type_json_all_contexts_roundtrip + test_bytes_type_view_encode_roundtrip to construct map values as Bytes.
• cargo test --workspace — 28 test suites pass, no failures.
• cargo clippy --workspace --all-targets -- -D warnings — clean.
• cargo fmt --all --check — clean.
• cargo check -p buffa --no-default-features (host no_std) — clean.
• task gen-wkt-types and gen-bootstrap-types produce no diff (neither set has map<K, bytes> fields).
• Pre-commit rust-code-reviewer + rust-api-ergonomics-reviewer agents per CLAUDE.md — no Critical findings; High (CHANGELOG entry) and Medium (docstring updates) addressed in this PR.

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