fix: recognize prebuilt circuit json by content

[ShiboSoftwareDev]

Summary

• recognize prebuilt Circuit JSON from a discriminated element array instead of requiring a special filename suffix
• preserve the existing canonical filename fallback for backward compatibility
• run check shorts successfully against shorted-board.json and render its PCB debug snapshot

Stack

• Reproduction: test: reproduce filename-dependent circuit json checks #4302
• This PR is based on the reproduction branch

Verification

• bun test tests/cli/check/check-shorts.test.ts (7 pass)
• bun test tests/cli/check (26 pass when run with localhost fixture access)
• bun run build
• bunx biome format cli/check/shared.ts tests/cli/check/check-shorts.test.ts
• fixed PCB SVG snapshot rendered and visually inspected

[github-actions]

This PR has been automatically marked as stale because it has had no recent activity. It will be closed if no further activity occurs.

[github-actions]

This PR was closed because it has been inactive for 1 day since being marked as stale.