workDir/output collision when a tutorial id equals another id + lang suffix

[jbrecht]

Flagged by the code-reviewer during PR #64 (parallel rendering). Severity: LOW — requires a pathological tutorial id; pre-existing, but #62's concurrency makes it actively dangerous.

Problem

Per-render paths are keyed by ${id}${suffix} where suffix = .${lang}:

• workDir = .forge/<id><suffix>, output = <outDir>/<id><suffix>.mp4 (packages/cli/src/render.ts).

So a tutorial with id foo rendered for lang es → .forge/foo.es, and a different tutorial whose id is literally foo.es rendered source-only → also .forge/foo.es. discoverTutorials rejects duplicate ids, but foo and foo.es are distinct ids, so both pass.

Serially this was a benign sequential overwrite; under --render-concurrency > 1 (#62) it's two renders writing the same dir/file simultaneously → corruption.

Note: this is distinct from the duplicate-language collision already fixed in #64 (langs are de-duped in buildRenderJobs).

Options

• Key the work dir by (id, lang) separately (e.g. .forge/<id>/<lang|_source>), or
• Detect the collision across the job list and error with a clear message.

Acceptance

• Two jobs can never resolve to the same workDir/output, regardless of id/lang naming.