snapshot restore crashes the app when an archive has duplicate panel IDs

[arzafran]

Problem

programa snapshot restore can kill the app instead of recovering it. If an archived snapshot in session-history/ contains two panels sharing a UUID, window reconstruction reaches Dictionary(uniqueKeysWithValues:) at Sources/Workspace+Persistence.swift:104, which traps on duplicate keys, and the app dies on a fatal error.

This is the command you reach for when a layout is already lost, so crashing here costs more than a crash anywhere else.

v2SnapshotRestore (Sources/TerminalController+Snapshot.swift:30-60) checks that the file decodes, that version matches the current schema, that windows is non-empty, and since #228 it clamps the window count. It never checks that panel IDs are unique within a workspace.

The same trap is reachable from startup restore reading session-<bundleId>.json, so this is not new behaviour. Snapshot restore adds a second door to it, and a worse one, because the input is a file the user may have been editing by hand while trying to recover.

Repro

1. Launch programa, let it save a session, then relaunch so the file is archived into ~/Library/Application Support/programa/session-history/.
2. Open the newest archive and duplicate one entry inside a workspace's panels array, keeping the id identical.
3. Run programa snapshot restore <id> for that archive.
4. The app terminates on a duplicate key fatal error instead of returning an error.

Fix sketch

Validate the decoded snapshot before handing windows to createMainWindow: reject workspaces whose panels contain repeated IDs and return an invalid_params error, or de-duplicate and keep the first. Putting it in one shared validator would cover the startup path at the same time.

Found while reviewing #228.