refactor(dart): deduplicate what PR #78 left over

[nikzen]

Summary

Follow-up to #78, which already resolved the bulk of a code review of nix/dart/ (shared OCI publishing, managed-file headers, fixture tests). This picks up the findings that remained, each as its own commit:

• vodozemac: the source.nix instantiation existed twice — once in the option declaration, once for the flake's packages. A shared mkSource helper means the two cannot drift apart.
• web image: the static-web-server feature-reduction override depends only on pkgs, so it now lives in the outer let instead of being restated per project and per call.
• web assets: the package name dart-web-assets<suffix> was written out four times across assets.nix and the build workflow. It is now a derived read-only option beside workflowId, for the same reason that one exists.
• project CLI: if flutter then "flutter" else "dart" was decided independently in the checks workflow and the pre-commit setup. It is now a derived read-only cli option, so every step makes the same choice.
• vodozemac module tidy-up (from a detailed follow-up review): the native bindings are instantiated through one mkBindings helper instead of two call sites, the licence lives on the shared source spec instead of being restated by both targets, frbVersion moved into the env block, and the devshell no longer lists the library — the store path runtime.env interpolates already makes the shell depend on it, which was verified by instantiating the fixture's devshell and checking its closure still holds famedly-vodozemac.

Verification

• nix build .#checks.<system>.dart-standards produces a store path byte-identical to the one built from main (7q6zfhikb987d2h1kjr1am9kmw91ibn6) — the refactor changes no generated output.

• dart-standards-workflows (actionlint + shellcheck) passes.

• The fixture devshell's derivation closure still contains famedly-vodozemac-0.5.0.drv after dropping the explicit devshell entry.

• treefmt reports no changes; all pre-commit hooks passed on commit.

• dart-vodozemac 0.5.0 → 0.7.1 (222d84c): the new release pins flutter_rust_bridge 2.12.0, whose build_web/executor.dart is byte-identical to 2.11.1's, so the wasm-pack flags stand; wasm-bindgen stays at 0.2.100, so the CLI pin does too. Both the native library and the WebAssembly module were built locally from the new release to verify the hashes and the frbVersion guard.