chore: code review cleanup — fixes, deps, docs

Fixed:
- CLI help typo: "metatdata" -> "metadata"
- Filter buffer OOM: check size before loading into memory

Changed:
- #[inline] on HTML escape helpers for hot path performance
- Replaced once_cell and lazy_static with std::sync::LazyLock
- Removed unused once_cell and lazy_static crate dependencies

Refactored:
- Added module-level doc to services/ module

Documentation:
- README.md: zstd is native not external, "none" -> "raw"
- DESIGN.md: current schema and meta plugins section
- CHANGELOG.md: Unreleased section populated

README.md

@@ -345,8 +345,8 @@ Items are compressed automatically on save. Default: LZ4.
 | `gzip` | Internal | Fast | Good |
 | `bzip2` | External | Slow | Better |
 | `xz` | External | Slowest | Best |
-| `zstd` | External | Fast | Good |
-| `none` | Internal | N/A | N/A |
+| `zstd` | Internal | Fast | Good |
+| `raw` | Internal | N/A | N/A |
 
 ```sh
 # Specify compression per item