[fylorn]

Three follow-ups on sync, on top of #357.

Default gateway key on join

Every installation generates a default gateway key. When the key that default_key points to has the same name on both sides and a different value, joining does not treat it as a conflict. The joining device takes the sync data's value, so it lines up with the devices already syncing:

• merge: that row is decided as overwrite; any decision given for it is ignored.
• use_remote: a pinned default key is no longer kept over the sync data's value.
• GatewayKeysChanged follows, so clients set up on the joining device are updated.

POST /sync/enable/preview returns the new SyncJoinPreview.auto: [{ item, action: "overwrite", reason: "default_key" }]. The row stays in rows, and the UI shows it as "use the sync data" with the note about clients. auto is also filled for initial configurations.

twcore sync enable

• When merging, each item that differs is asked: u use the sync data (default), k keep this device's version (the prompt says it replaces the other devices' one), and r keep both where a new name is suggested. Before this, Enter skipped every conflict, which uploaded this device's values (its default key included) over the other devices'.
• The device name defaults to the computer name: scutil --get ComputerName (then LocalHostName) on macOS, the host name on Linux, COMPUTERNAME on Windows. Before this, macOS fell back to server.

iCloud placeholders (macOS 13 and earlier)

Reading the real name behind a legacy .name.icloud placeholder does not download it.

• The folder location now asks iCloud to download it with NSFileManager startDownloadingUbiquitousItemAtURL:error: (the documented API, available since 10.7; brctl download is gone on current macOS).
• The call goes through the system osascript (JavaScript for Automation), with the path passed as an argument, at most every 5 minutes per file. There are no native calls in core.
• A placeholder reads as empty and counts as "arriving", not "unreadable". It does not count toward the 30-minute skip and is skipped only after a day. The keyring gets the same treatment.

Tests

• tw-control/tests/sync_regressions.rs: merge join adopts the default key even with a "skip" decision; a fresh device with taken-over clients adopts it on use_remote; GatewayKeysChanged is emitted; the other device's key is unchanged. The earlier merge-join test is updated to the new rule.
• tw-sync/tests/regressions.rs: placeholders are still waited for after 30 minutes and skipped after a day. A macOS-only unit test runs the download request script (it answers false for a file outside iCloud). The device-name helper has unit tests.
• Local: cargo test --workspace 3828 passed, 0 failed; cargo clippy --workspace --all-targets -D warnings clean; cargo fmt --check clean; scripts/smoke.sh 84 passed, 0 failed. The platform-specific functions were also linted for the Linux and Windows targets in a scratch crate.
• Manual run with two real cores and a folder location: twcore sync enable offered the computer name as the device name, printed the default-key note, asked about the one differing upstream (default: use the sync data), and adopted the sync data's default key. The other device's key stayed as it was.
• Not verified on a real macOS 12/13 iCloud Drive: the download request was only checked to run.

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