refactor: share runtime id generation

[Genmin]

Summary

• add a shared runtime generateId() helper
• use it for message IDs and crash-record filename suffixes
• export the helper from the runtime package
• add focused coverage for the 16-character lowercase hex contract

Closes #61

Validation

• temp-installed TypeScript/Bun types: tsc -p packages/runtime/tsconfig.json --noEmit
• temp-installed Biome: biome check packages/runtime/src/id.ts packages/runtime/src/id.test.ts packages/runtime/src/message.ts packages/runtime/src/crash.ts packages/runtime/src/index.ts
• git diff --check

Note

• Bun is not installed in this local environment, so I could not run bun test; the added test follows the existing bun:test style and is covered by typecheck.

[Hilo-re]

Clean extraction. Same logic, zero duplication — exactly what the issue asked for. We'll confirm bun test passes on our end before merging.

[Hilo-re]

Code looks right — clean extraction, test follows the existing pattern, export placed correctly in index.ts. One thing before I merge: can you confirm bun test passes locally? The CI doesn't gate on it, but the project relies on Bun for testing and it's worth verifying the new test actually runs.

[Hilo-re]

Missed that you already noted Bun isn't available locally — that's fine. We'll run the tests on our end before merging. Should be quick.