Fine. Rename binary I/O types to BinaryReader/Writer

Replace the generic names `SummaryDataReader` and `BufferedSink` with
clearer `BinaryReader` and `BinaryWriter`. The new names reflect their
actual roles (reading/writing the analyzer’s binary formats), reduce
confusion with “summary” artifacts, and make call sites easier to scan.
No encoding or behavioral changes are intended.

Key changes
- Introduce `BinaryReader` and `BinaryWriter` and update class docs.
- Rename constructor, method, and factory signatures across the codebase
  to use the new types (e.g., `read(...)`, `write(...)`, `fork(...)`).
- Update extensions to match the new names (`BinaryReaderExtension`,
  `BinaryWriterExtension`) and migrate helper methods accordingly.
- Adjust `string_table.dart` to take `BinaryWriter` and update `_writeWtf8`.
- Update readers/writers throughout analyzer components:
  - library context/diagnostics, unlinked data, manifests, IDs, items,
    types, requirements, and informative data.
  - utilities for `EnumSet.read`/`write`.
  - summary2 bundle reader/writer and package bundle format.
- Keep `_SummaryDataWriter` as an internal adapter now extending
  `BinaryWriter`.

Compatibility
- Binary format is unchanged; this is a pure rename refactor.
- Endianness, variable-length encodings, and offsets remain identical.

Change-Id: I97c07a708c1bd649084fc78a7c60c583964109ca
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/450950
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>