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>

flake.nix

@@ -16,19 +16,36 @@
           overlays = [ rust-overlay.overlays.default ];
         };
 
-        rustToolchain = pkgs.rust-bin.stable.latest.default.override {
-          extensions = [ "rust-src" "rust-analyzer" "clippy" "rustfmt" ];
+        # Pinned rather than `stable.latest`, so the toolchain only moves
+        # when this line does. `latest` drifts with the flake inputs, which
+        # means a lint can start failing on an unrelated change.
+        devVersion = "1.99.1";
+
+        # Must match `rust-version` in Cargo.toml. The floor comes from the
+        # dependency graph, not from this crate's own code: reqwest pulls in
+        # url -> idna -> idna_adapter -> icu_*, and the icu crates require
+        # 1.88. Check with:
+        #   cargo metadata --format-version 1 \
+        #     | jq -r '.packages[] | select(.rust_version) | .rust_version' \
+        #     | sort -V | tail -1
+        msrvVersion = "1.88.0";
+
+        toolchainFor = version: extensions:
+          pkgs.rust-bin.stable.${version}.default.override { inherit extensions; };
+
+        rustToolchain = toolchainFor devVersion [ "rust-src" "rust-analyzer" "clippy" "rustfmt" ];
+        msrvToolchain = toolchainFor msrvVersion [ ];
+
+        shellFor = toolchain: pkgs.mkShell {
+          packages = [ toolchain pkgs.pkg-config pkgs.openssl ];
+          RUST_SRC_PATH = "${toolchain}/lib/rustlib/src/rust/library";
         };
       in
       {
-        devShells.default = pkgs.mkShell {
-          packages = [
-            rustToolchain
-            pkgs.pkg-config
-            pkgs.openssl
-          ];
+        devShells.default = shellFor rustToolchain;
 
-          RUST_SRC_PATH = "${rustToolchain}/lib/rustlib/src/rust/library";
-        };
+        # Verifies the declared MSRV still builds:
+        #   nix develop .#msrv --command cargo test
+        devShells.msrv = shellFor msrvToolchain;
       });
 }