chore(nix): wasm32-wasip2 toolchain groundwork (hu stack 1/9)

[YuanYuYuan]

Summary

First of a stacked series splitting the large hu (hiroz-union) skeleton PR (#231) into small, individually-reviewable, individually-green pieces. This PR carries only the Nix toolchain groundwork the later plugin work needs — no crates, no CLI, no behavior change.

Key Changes

• flake.nix: introduces rustToolchainWasm (the existing pure-Rust toolchain plus the wasm32-wasip2 target) and wires it into the pureRust-ci shell so CI can build the WASM plugins inline, plus a new pureRust-wasm dev shell for building plugins locally with the full dev toolchain. The wasm sysroot (~100–500 MB) is fetched only when a wasm-capable shell is entered; the default pureRust shell is unchanged and stays lean.
• .cargo/config.toml: adds a single [alias] entry, cargo hu-plugins, as a convenience for building the plugin workspace. It changes no build flags, targets, or RUSTFLAGS for normal cargo invocations.
• .gitignore: ignore local .cargo/ state while keeping the committed config.toml.
• flake.lock: routine input bump.

What fails without this

Nothing yet — this is groundwork. The later child PRs (plugin host, hu-meter/hu-monitor, WASM plugin CI job) require the wasm32-wasip2 target to be available in the CI shell; this PR makes it available without changing any current build.

Breaking Changes

None.

Stacked PR series

This is PR 1 of 9. Base: main. Subsequent children stack on this branch. The tip of the stack reconstructs the current dev/pr-hu-1-skeleton exactly.

[Copilot]

Pull request overview

Introduces Nix + Cargo configuration groundwork for upcoming hu WASM plugin work by adding a WASM-capable Rust toolchain option and related dev/CI shells, plus a few quality-of-life and caching-oriented updates.

Changes:

• Extend the Rust toolchain options with a wasm32-wasip2-enabled variant and add a pureRust-wasm dev shell; wire the WASM toolchain into CI-oriented shells.
• Improve ROS shell env construction (per-package AMENT_PREFIX_PATH) and restrict generated per-distro shells to distros present in the pinned nix-ros-overlay.
• Add a Cargo alias and adjust .gitignore to ignore local .cargo/* while keeping committed config; bump flake.lock and add crane.

Reviewed changes

Copilot reviewed 2 out of 4 changed files in this pull request and generated 3 comments.

File	Description
flake.nix	Adds WASM-capable Rust toolchain/shells, per-package AMENT prefix handling, and introduces crane-based packaging.
flake.lock	Adds crane input and bumps pinned Nix inputs.
.gitignore	Ignores local .cargo/* while keeping .cargo/config.toml committed.
.cargo/config.toml	Adds a cargo hu-plugins convenience alias for the upcoming plugin workspace.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 4 changed files in this pull request and generated 2 comments.

Comments suppressed due to low confidence (2)

flake.nix:595

• The flake now defines a packages.hu (and packages.default) that builds -p hiroz-union --bin hu, but this repo state doesn’t contain a hiroz-union crate or a hu binary yet (only crates/hiroz-console exists). As a result, nix build .#hu / nix build will fail when invoked, which undermines the goal of this PR being independently green groundwork.

              nativeBuildInputs = [
                pkgs.pkg-config
                pkgs.protobuf
              ];
              cargoExtraArgs = "-p hiroz-union --bin hu";
              doCheck = false;

.cargo/config.toml:7

• cargo hu-plugins currently points at crates/hiroz-union/plugins/Cargo.toml, but that manifest (and the hiroz-union workspace) is not present in this PR. This makes the new alias fail immediately if used, and the comment also references a docs file that isn’t in-tree yet.

# Build the hu WASM plugins (meter + monitor) from the repo root without
# remembering the manifest path and target. Append --release for an optimized
# build. See docs/tools/hu-plugins.md.
hu-plugins = "build --manifest-path crates/hiroz-union/plugins/Cargo.toml --target wasm32-wasip2 --workspace"

[Copilot]

Pull request overview

Copilot reviewed 2 out of 4 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (2)

.cargo/config.toml:7

• The new hu-plugins alias points at crates/hiroz-union/plugins/Cargo.toml, but that path does not exist in the current workspace (and hiroz-union is not a workspace member yet). As-is, cargo hu-plugins will fail immediately from the repo root.

# Build the hu WASM plugins (meter + monitor) from the repo root without
# remembering the manifest path and target. Append --release for an optimized
# build. See docs/tools/hu-plugins.md.
hu-plugins = "build --manifest-path crates/hiroz-union/plugins/Cargo.toml --target wasm32-wasip2 --workspace"

flake.nix:595

• cargoExtraArgs = "-p hiroz-union --bin hu" references a workspace package that does not exist in this PR (the workspace currently contains hiroz-console, not hiroz-union). This makes the new packages.hu output non-buildable on this branch.

              cargoExtraArgs = "-p hiroz-union --bin hu";