Named registers ("a–"z) with a yank ring

[ooyeku]

Stem has one unnamed clipboard (src/kernel/core.zig, src/kernel/commands/edit_commands.zig). Add vim-style named registers plus a numbered yank ring, with a register picker in the command palette.

Stretch (separate PR): persist registers per-project through the checkpoint pipeline so yanks survive a crash — see how sessions checkpoint in src/kernel/core.zig (writeRecoverySnapshot).

Roadmap: 0.13.0 (docs/roadmap.md).