[Bug]: cargo metadata compatibility warning repeated in every CI metadata validation step

## Summary

The upstream CI logs for `main` show a cargo toolchain warning repeated in every "Verify ... metadata" step. Evidence from GitHub Actions run [33035362609](https://github.com/GCWing/BitFun/actions/runs/33035362609) (also present in runs 33050974141, 33047041028, and the latest snapshot run 33057289117):

```
warning: please specify `--format-version` flag explicitly to avoid compatibility problems
```

It is emitted by the three `cargo metadata --locked --no-deps` invocations in `.github/workflows/ci.yml` (the "Verify committed Cargo metadata", "Verify committed release metadata", and "Verify projected release metadata" steps). It appears once per runner in the Rust Build Check jobs and again in the frontend job, i.e. it pollutes every CI run of `main`.

## Root Cause Analysis

- Newer cargo releases print this compatibility warning whenever `cargo metadata` is invoked without an explicit `--format-version` flag. The workflow calls omit the flag, so every run produces the warning even though the invocation itself succeeds.
- This is a CI-configuration-layer issue, not a code warning: no source change is involved.

## Proposed Fix

Add `--format-version 1` explicitly to the three `cargo metadata --locked --no-deps` invocations in `.github/workflows/ci.yml`, so:

- the compatibility warning disappears from the CI logs;
- the invocation pins its machine-readable output contract explicitly, protecting against future cargo default changes.

No workflow semantics change: the same metadata is validated with the same lockfile, just with the format version made explicit.

Open .github/workflows/ci.yml and inspect the three cargo metadata --locked --no-deps invocations in the metadata validation steps. Add the explicit format-version option described in the issue, then run the relevant CI workflow or metadata validation commands. Done means all three checks still pass and the compatibility warning no longer appears in the logs.

索引モデルが issue の本文から書いたものです。