build: declare Rust 1.92 as the MSRV and check it in CI

rust-version = "1.92" in [workspace.package], inherited by every crate.
1.92 is the floor: wgpu (clawhdf5-gpu) requires it, and the whole
workspace, Python bindings included, checks cleanly on it. ci-test.sh
reads the version from Cargo.toml and checks the workspace on exactly
that toolchain, so the manifests and the README badge cannot drift from
what actually builds. The badge said 1.75, below edition 2024's own
floor.

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

CHANGELOG.md

@@ -11,6 +11,8 @@
   overrides zlib-rs wherever it is on.
 - **A truncated deflate chunk is now an error.** It used to read back short,
   with no error.
+- **Minimum supported Rust is 1.92**, now declared in every crate's
+  `rust-version` and checked in CI.
 - **New stores use the int8 vector index by default.**
   `MemoryConfig::quantized_index` now defaults to `true`: a quarter of the
   index memory, builds 1.8x (x86-64) and 2.3x (Raspberry Pi 5) faster, and
@@ -32,6 +34,8 @@
   no `std`, cannot detect SIMD at runtime, and inflates 3.5x slower; the
   workspace builds flate2 with `default-features = false`, which had been
   switching it off.
+- `rust-version = "1.92"` for the whole workspace (the floor: `wgpu` requires
+  it), and CI checks the workspace on exactly that toolchain.
 - CI keeps zlib-ng building and tested; the arm64 job no longer needs cmake.
 
 ### Correctness