fix: unify playlist item short ID hash algorithm to SHA256

[proboscis]

Summary

• Replace unstable DefaultHasher with stable SHA256-based hashing in PlaylistItem::short_id()
• Ensure consistency between runbox playlist show and runbox run <short_id>
• All playlist item short IDs are now stable across Rust versions

Problem

Playlist item short IDs displayed by runbox playlist show couldn't be used with runbox run <id>:

➜ runbox playlist show game_run
# Shows short ID: 35a2287e

➜ runbox run 35a2287e
Error: No runnable found matching '35a2287e'

Root Cause

Two different hashing algorithms were used:

• playlist.rs used DefaultHasher (UNSTABLE across Rust versions)
• runnable.rs used SHA256 (STABLE)

Solution

Unified on SHA256-based stable hash by:

1. Exporting stable_short_id() from runnable.rs
2. Updating PlaylistItem::short_id() to use the same algorithm

Acceptance Criteria Evidence

All tests pass (121 core tests + 18 integration tests):

cargo test --package runbox-core
test result: ok. 121 passed; 0 failed; 0 ignored

cargo test --package runbox-cli playlist_run test_list
test result: ok. 18 passed; 0 failed; 0 ignored

• runbox playlist show displays same short IDs as runbox list --type playlist
• runbox run <short_id> works with IDs shown by runbox playlist show
• All playlist item short IDs are stable across Rust versions (SHA256-based)
• Tests pass

Closes ISSUE-022