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>

Cargo.toml

@@ -2,6 +2,11 @@
 name = "invoiceninjars"
 version = "0.1.0"
 edition = "2021"
+# Set by the dependency graph rather than by this crate's own code: reqwest
+# pulls in url -> idna -> idna_adapter -> icu_*, and the icu crates require
+# 1.88. Verified by running the test suite against it; see the `msrv`
+# devShell in flake.nix.
+rust-version = "1.88"
 description = "An async Rust client for the Invoice Ninja v5 API"
 license = "MIT"
 repository = "https://github.com/bryant/invoiceninjars"