chore(deps): consolidate 12 dependency PRs and fix the nightly clippy break blocking them all

[fredclausen]

Consolidates all 12 open dependency PRs into one branch and fixes the single root
cause that was holding every one of them red.

Supersedes and closes: #43 #44 #45 #46 #47 #48 #49 #50 #51 #52 #53 #54

Root cause

All 12 PRs failed identically on Check ubuntu-latest nightly and
Check macos-latest nightly. None of them failed because of their own content:

error: enum with no variants
   --> crates/fredshell-core/src/lib.rs:117:1
117 | pub enum BuiltinError {}
    = note: `-D clippy::empty-enums` implied by `-D clippy::nursery`

clippy::empty_enums is a nursery lint, and the workspace sets
nursery = { level = "deny" }. It began firing when nightly clippy picked the
lint up, so every branch in the repo went red at once — including main, which
still has the same code. Stable was and is unaffected.

I reproduced this against today's nightly (1.100.0-nightly, clippy 0.1.100)
and confirmed empty_enums is the only error; nothing else has accumulated
in the intervening weeks.

Changes

1. fix: allow clippy::empty_enums on BuiltinError

One line. BuiltinError is uninhabited deliberately — its doc comment reserves
the slot for future builtins (read, wait) whose failure modes are richer
than an exit status, and CoreError::Builtin already wraps it. The emptiness is
a temporary property of an intentional design, so the lint is wrong at this site.
This mirrors the pre-existing #[allow(clippy::uninhabited_references)] on the
Display impl three lines below, which exists for exactly the same reason.

2. chore(flake): add devShells.nightly

CI gates on nightly, but devShells.default only carried the stable toolchain —
so a nightly-only break could not be reproduced locally at all. That gap is why
one lint sat undiagnosed across 12 PRs for weeks. I had to build a throwaway
nix shell to find it.

The new shell deliberately omits extraDev and corePkgs: those carry the
stable rustc/cargo/clippy plus pre-commit tooling, and mixing them with nightly
would put two toolchains on PATH and trip the E0514 failure already documented
on the default shell. Trade-off is explicit in a comment — no pre-commit hooks in
the nightly shell, so commit from default and use nightly only to run
cargo xtask check.

nix develop --impure .#nightly --command cargo xtask check

3. The 12 dependency bumps

Area	Updates
Rust crates	base64 0.23.1, clap 4.6.6, syn 3.0.5, plus Cargo.lock maintenance
Actions	determinate-nix-action, dtolnay/rust-toolchain, Swatinem/rust-cache 2.9.2, taiki-e/install-action 2.87.12
Flake inputs	nixpkgs, nixpkgs-reference 88ae382, precommit, rust-overlay

Notes on the merge

• #52, #53, #54 (the three update-* flake-input PRs) turned out to be
  fully superseded — renovate/lock-file-maintenance had already advanced
  every input past what they proposed. They are included in the history but
  contribute nothing on top.
• The one genuine conflict was flake.lock between lock-file-maintenance and
  the update-* branches. Rather than hand-pick hunks across 12 conflicted
  blocks, I resolved it by regenerating the lock with nix flake update, which
  is the only resolution guaranteed internally consistent. All inputs are now at
  current rather than at four different stale points.
• Cargo.lock needed one reconciliation the merges could not do themselves: a
  stale syn 3.0.3 entry under wasm-bindgen resolved to 3.0.5.

Verification

cargo xtask check (fmt + clippy -D warnings + machete + test + doc) run
locally in both toolchains:

Toolchain	Result
stable (dev shell)	exit 0
nightly 1.100.0 / clippy 0.1.100	exit 0

pre-commit run --all-files — all 25 active hooks pass.
nix flake check — all outputs evaluate, including both dev shells.

Deliberately not done

CI posture is unchanged. nursery + pedantic at deny, on nightly, with
-D warnings, as a required check, means clippy's release schedule can turn
the whole repo red with no code change — which is precisely what happened here.
Fixing this one lint does not prevent the next one. Making the nightly leg
advisory was considered and explicitly deferred; worth revisiting if this recurs.

Summary by CodeRabbit

• Developer Experience

  • Added a dedicated nightly Rust development environment with Clippy, rustfmt, and cargo-machete support.
  • Preserved the existing stable development environment and its reference-toolchain configuration.
  • Updated the nightly environment to use an isolated build directory.

• Maintenance

  • Refreshed development dependencies and pinned tooling revisions.
  • Updated the Nix package reference used by development environments.
  • Added a targeted lint configuration for an intentionally uninhabited error type.

[coderabbitai]

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• crates/fredshell-core/src/lib.rs
• flake.nix

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[Copilot]

🔵 Needs a closer look

A required reference-document update remains unresolved, and the broad dependency and flake refresh warrants human review.

Pull request overview

Consolidates dependency updates, fixes the nightly Clippy failure, and adds a reproducible nightly Nix shell.

Changes:

• Allows the intentional empty BuiltinError enum.
• Adds an isolated nightly development shell.
• Updates Rust, Nix, GitHub Actions, and lockfiles.

File summaries

File	Description
flake.nix	Adds the nightly development shell.
flake.lock	Refreshes pinned Nix inputs.
crates/fredshell-core/src/lib.rs	Allows the intentional empty enum.
Cargo.toml	Updates direct dependencies.
Cargo.lock	Refreshes dependency resolution.
.github/workflows/ci.yml	Updates pinned action revisions.

Review details

Suppressed comments (1)

flake.nix:19

• This changes the authoritative nixpkgs-reference revision, but tests/spec/REFERENCE.md still declares aec71e3... (and the old pin date). The repository's upgrade policy requires the machine-readable [reference] block to be updated with every flake pin bump; please update that document and re-record/inspect fixtures if the new Bash or coreutils versions differ.

    nixpkgs-reference.url = "github:nixos/nixpkgs/88ae3822eb8aec31f12a4a1895cb064413511177";

• Files reviewed: 4/6 changed files
• Comments generated: 0
• Review effort level: Lite

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