fix: unify playlist item short ID hashing to SHA256

[proboscis]

Summary

• Fixes ISSUE-039: Playlist item short IDs displayed by runbox playlist show now match the IDs used by runbox run <id>

Problem

Two different hashing algorithms were used for playlist item short IDs:

• playlist.rs: DefaultHasher (unstable across Rust versions)
• runnable.rs: SHA256 (stable)

This caused runbox run <short_id> to fail with "No runnable found" when using IDs from runbox playlist show.

Changes

• Export stable_short_id from runnable.rs
• Update PlaylistItem::short_id() to use the same SHA256-based algorithm as Runnable::PlaylistItem.short_id()
• Both now use format: playlist_item\0{playlist_id}\0{index}\0{template_id}

Acceptance Criteria Verification

runbox playlist show displays same short IDs as runbox list --type playlist

=== runbox playlist show pl_game_runtimes ===
IDX   SHORT      TEMPLATE        LABEL
0     1bd3a756   game_cli        CLI - Text-based terminal
1     64b65e8c   game_tui        TUI - Rich Textual interface
2     a1067dcd   game_pyg        Pygame - Graphical interface

runbox run <short_id> works with IDs shown by runbox playlist show

=== runbox run 1bd3a756 --dry-run ===
┌───────────────────────────────────────────────────────┐
│ PLAYLIST ITEM: pl_game_runtimes[0] "CLI - Text-based terminal"│
│ Template: tpl_game_cli                                 │
└───────────────────────────────────────────────────────┘
(successfully resolved and would execute)

All tests pass

• 121 core tests pass
• 53 CLI playlist tests pass

Closes ISSUE-039