Pin the Rust toolchain and declare an MSRV of 1.88

The dev shell tracked `stable.latest` and CI used `@stable`, so nothing
pinned a version and nothing declared a floor. A consumer on an older
toolchain hit a confusing error deep in a dependency rather than cargo
saying the version was too old, and a new stable release could turn a
lint into an unrelated red build.

- `rust-version = "1.88"` in Cargo.toml. The floor comes from the
  dependency graph, not this crate's code: reqwest pulls in url -> idna
  -> idna_adapter -> icu_*, and the icu crates require 1.88. Verified by
  running the full suite against 1.88, not inferred.
- flake.nix pins the dev toolchain to 1.99.1 and adds an `msrv` devShell,
  so the floor can be rechecked with
  `nix develop .#msrv --command cargo test`.
- CI pins the main job to the same 1.99.1 and adds a job building on
  1.88. Lint and format stay on the pinned toolchain only, since their
  output varies by release.

Note the tradeoff: with CI pinned, a future stable that breaks the build
will not be noticed until the pin moves. Worth adding a non-blocking
`stable` job if that matters.

CLAUDE.md records the three places that must agree when either version
moves, and how to recheck the floor after a dependency bump.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -2,6 +2,8 @@
 
 An async Rust client for the [Invoice Ninja](https://www.invoiceninja.com/) v5 API.
 
+Requires Rust 1.88 or newer.
+
 ## Development
 
 Enter the dev shell (Rust toolchain via `rust-overlay`):
@@ -10,6 +12,14 @@ Enter the dev shell (Rust toolchain via `rust-overlay`):
 nix develop
 ```
 
+The dev toolchain is pinned in `flake.nix` rather than tracking latest
+stable, so it only moves deliberately. To check the crate still builds on
+the declared minimum of 1.88:
+
+```sh
+nix develop .#msrv --command cargo test
+```
+
 ## Usage
 
 ```rust