docs: crate READMEs describe each crate as it is today

Every crate under crates/ now has a README (android, bench, cli, napi and
wasm had none), each saying what the crate is, its main types and
functions (names checked against the code), its cargo features with
defaults and which ones build C (checked with `cargo tree`), and links to
the top-level docs.

Corrections to the old stubs:
- clawhdf5-derive: the derive is `H5Type`, not `HDF5Type`, and it needs
  clawhdf5-format as a dependency.
- clawhdf5-filters: deflate backends only, and no library crate depends
  on it; the filter pipeline and every other codec are in -format.
- clawhdf5-gpu: vector distance compute, not I/O; not used by
  HDF5Memory::search.
- clawhdf5-io: MpiVol is root-read + broadcast, not collective MPI-IO.
- clawhdf5-ann: from_hdf5/search(q, k) did not exist; load_from_hdf5 and
  search(q, k, ef).
- clawhdf5-accel: checksum::crc32_simd did not exist; the SSE4 and wasm
  backends are reported but run the scalar kernels.
- clawhdf5-gpu: the old example called l2_distances, which does not
  exist (l2_search).
- clawhdf5-agent: it described a "vector store" with "GPU acceleration";
  it now covers HDF5Memory, search options, WAL, signing, the graph.
- crates.io/docs.rs badges removed and `cargo install <crate>` replaced:
  nothing is published; depend on git.
- fuzz: the opt-in CLAWHDF5_FUZZ_SECONDS smoke run in ci-test.sh.
- tools: the FileEditor interop tests that live in this crate.
- remote, py: license, other front ends, limits, File.mode/flush/chunks.

The Rust examples of the facade, format, filters, accel, ann, derive and
agent READMEs were compiled and run as tests (netcdf4, gpu and remote
compiled only) in a scratch crate; the CLI example was run.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/clawhdf5-derive/README.md

@@ -1,28 +1,50 @@
 # clawhdf5-derive
 
-[![crates.io](https://img.shields.io/crates/v/clawhdf5-derive.svg)](https://crates.io/crates/clawhdf5-derive)
-[![docs.rs](https://docs.rs/clawhdf5-derive/badge.svg)](https://docs.rs/clawhdf5-derive)
+`#[derive(H5Type)]`: maps a Rust struct with named fields to an HDF5
+compound datatype. The derive generates three inherent methods:
 
-Derive macros for clawhdf5 HDF5 traits.
+- `hdf5_datatype() -> clawhdf5_format::datatype::Datatype` — the
+  `Datatype::Compound` (members in field order, packed, little-endian);
+- `to_bytes(&self) -> Vec<u8>` — one element in that layout;
+- `from_bytes(&[u8]) -> Self` — the reverse (panics if the slice is shorter
+  than the compound).
 
-## Features
+Supported field types: `f32`, `f64`, `i8`–`i64`, `u8`–`u64`, `bool`
+(stored as `u8`) and fixed-size arrays `[T; N]` of those numeric types.
+Tuple structs, enums and nested structs are refused at compile time.
 
-- `#[derive(HDF5Type)]` for automatic HDF5 datatype mapping
-- Struct-to-compound-type derivation
+The generated code names `clawhdf5_format`, so the crate using the derive
+must depend on [`clawhdf5-format`](../clawhdf5-format/README.md) too. Not
+on crates.io yet:
 
-## Usage
+```toml
+[dependencies]
+clawhdf5-derive = { git = "https://git.redclaw.dev/quantumclaw/clawhdf5" }
+clawhdf5-format = { git = "https://git.redclaw.dev/quantumclaw/clawhdf5" }
+```
+
+## Example
 
 ```rust
-use clawhdf5_derive::HDF5Type;
+use clawhdf5_derive::H5Type;
+use clawhdf5_format::datatype::Datatype;
 
-#[derive(HDF5Type)]
+#[derive(H5Type, Debug, PartialEq)]
 struct Point {
-    x: f64,
-    y: f64,
-    z: f64,
+    id: u32,
+    pos: [f64; 3],
+    valid: bool,
 }
+
+let p = Point { id: 7, pos: [1.0, 2.0, 3.0], valid: true };
+let bytes = p.to_bytes();
+assert_eq!(bytes.len(), 4 + 24 + 1);
+assert_eq!(Point::from_bytes(&bytes), p);
+assert!(matches!(Point::hdf5_datatype(), Datatype::Compound { size: 29, .. }));
 ```
 
+Tests: `crates/clawhdf5-format/tests/derive_tests.rs`.
+
 ## License
 
 MIT