fix(#2550): make pedantic clippy lints a hard error, fix stale checks.nix comment

The clippy check's comment described `-D warnings -A clippy::pedantic` — the
`-A` half dropping pedantic from the CI gate — but the args were only
`-D warnings`, so pedantic was hard-denied contrary to the doc. Operator
call: pedantic should be gated. Encode that as the single source of truth:
set the workspace lint `pedantic = deny` (errors locally and in CI), and
rewrite the checks.nix comment to match. Args unchanged; `-D warnings` still
gates rustc + non-pedantic clippy warnings. No new failures — the tree was
already pedantic-clean under CI's `-D warnings`, which denied pedantic.

Cargo.toml

@@ -22,7 +22,11 @@ edition = "2024"
 version = "0.1.0"
 
 [workspace.lints.clippy]
-pedantic = { level = "warn", priority = -1 }
+# Pedantic is a hard error (locally + in CI): we want pedantic lints
+# gated, so a toolchain bump that adds a new one reds the build until the
+# code is updated rather than sliding in unnoticed. Priority -1 keeps the
+# specific allows below winning over the group.
+pedantic = { level = "deny", priority = -1 }
 # Tolerated stylistic pedantic lints (noisy, not actionable).
 missing_errors_doc = "allow"
 missing_panics_doc = "allow"

nix/checks.nix

@@ -19,17 +19,14 @@ in
   # shared `cargoArtifacts` (deps already built) and runs
   # `cargo clippy --workspace --all-targets` directly.
   #
-  # `-D warnings` makes the default/correctness/style lints a
-  # hard CI gate. `-A clippy::pedantic` then drops the pedantic
-  # group from that gate: pedantic is the "extra, opinionated"
-  # group the clippy team grows freely, so denying it means
-  # every toolchain bump that adds a new pedantic lint breaks CI
-  # with zero code changes. The `pedantic = warn`
-  # workspace lint (Cargo.toml) keeps it as advisory signal in
-  # local `cargo clippy` — it just no longer blocks the build.
-  # (`-A` rather than `-W` here: `-W clippy::pedantic` would
-  # re-enable the specific pedantic lints the workspace lints
-  # table allows, e.g. `must_use_candidate`.)
+  # `-D warnings` makes every rustc/clippy warning a hard CI gate.
+  # Pedantic is gated too, deliberately: the workspace lint table
+  # (Cargo.toml) sets `pedantic = deny`, so pedantic lints are errors
+  # both locally and here. We want that — a toolchain bump that adds a
+  # new pedantic lint reds the build until the code is updated, rather
+  # than sliding in unnoticed. (The lint table allows a few noisy
+  # pedantic lints explicitly, e.g. `must_use_candidate`; those keep
+  # their allow via higher priority.)
   clippy = craneLib.cargoClippy {
     src = cleanSrc;
     inherit cargoArtifacts nativeBuildInputs;