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

Cargo.toml

@@ -35,7 +35,6 @@ hyper = { version = "1.0", features = ["full"] }
 http-body-util = "0.1"
 inventory = "0.3"
 is-terminal = "0.4"
-lazy_static = "1.5"
 libc = "0.2"
 local-ip-address = "0.6"
 log = "0.4"
@@ -45,7 +44,6 @@ magic = { version = "0.13", optional = true }
 infer = { version = "0.19", optional = true }
 tree_magic_mini = { version = "3.2", optional = true }
 nix = { version = "0.30", features = ["fs", "process"] }
-once_cell = "1.21"
 comfy-table = "7.2"
 pwhash = "1.0"
 regex = "1.10"