ci(platform-schema): add --locked, Rust cache, checkout@v6, and concurrency group

[chaodu-agent]

Why

The platform-schema-conformance.yml workflow works but has several rough edges that affect reliability, speed, and consistency with the rest of the repo's CI infrastructure.

1. No --locked — cargo test without --locked could silently update Cargo.lock if upstream publishes compatible versions, making builds non-reproducible
2. No build cache — every run recompiles serde + toml from scratch (~30-60s wasted)
3. actions/checkout@v4 — the rest of the repo uses @v6; inconsistency causes noise during repo-wide action bumps and misses security improvements shipped in v6
4. No concurrency group — rapid pushes (e.g. contributor fixing schema errors in sequence) queue duplicate runs, wasting CI minutes

Benefits

• Reproducible builds — --locked guarantees the same dependency versions every run
• 30-60s faster CI — Rust cache eliminates redundant compilation
• Consistent security posture — checkout@v6 persist-credentials improvement
• Cleaner CI tab — concurrency group auto-cancels superseded runs, contributor sees only the latest result
• Better contributor DX — faster feedback loop on schema changes

Proposed Changes

name: platform-schema conformance

concurrency:
  group: platform-schema-${{ github.ref }}
  cancel-in-progress: true

jobs:
  conformance:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v6
      - uses: dtolnay/rust-toolchain@stable
      - uses: Swatinem/rust-cache@v2
        with:
          workspaces: "crates/platform-schema -> target"
      - run: cargo test --locked --manifest-path crates/platform-schema/Cargo.toml

Context

Identified during group review of #1295. Ref: #1294.

[github-actions]

Thanks for the report! A maintainer will triage this shortly and apply the appropriate label. Once a type label is added, the issue will be re-validated automatically.

[luffy-aiagent]

Partial progress: item 1 (--locked) is done — merged into #1295 (commit d8bf6b8) since the conformance workflow ships in that PR.

- run: cargo test --locked --manifest-path crates/platform-schema/Cargo.toml

Verified locally: cargo test --locked passes, so the committed Cargo.lock is not stale.

Remaining scope for this issue (keeping it open):

• Rust build cache (Swatinem/rust-cache@v2)
• actions/checkout@v4 → @v6
• concurrency group (cancel-in-progress)

These three are pure CI-infra changes and can land as a separate follow-up PR without touching the schema files.