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]>

CLAUDE.md

@@ -31,7 +31,8 @@ Cargo workspace with 16 crates under `crates/` (plus `libaec-sys`, an internal F
   pure-Rust zlib-rs (`fast-deflate` opts into zlib-ng, which needs cmake).
   `ci-test.sh` fails if a C-building crate enters the core crates' default
   tree. flate2 must keep `runtime_detection` with zlib-rs — without it zlib-rs
-  loses SIMD and inflates 3.5x slower.
+  loses SIMD and inflates 3.5x slower. MSRV is 1.92 (`rust-version`, checked
+  in CI).
 - HNSW vector index for semantic similarity search over agent memories — the
   `clawhdf5-agent` `hnsw` feature is **on by default**, so `hybrid_search` uses
   the approximate `clawhdf5-ann` index for the vector stage (the index mirrors