Sync: oversized preferences are left out instead of failing every round; CLI explains keep and keep-both

[fylorn]

Two small follow-ups on sync, on top of #358.

Oversized general preferences

Preferences over CLIENT_PREFS_MAX cannot be written on this device. They used to make write_prefs refuse the write, which failed the whole apply on every round. The view stayed "idle" with an error and, after two hours, reported the location as unreachable.

Now:

• When the sync data's preferences are too large they are held: this device keeps its own, uploaded versions keep the sync data's (this device's are never pushed over them), and everything else syncs. The same applies when the merged preferences come out too large, and on join.
• SyncView.notices (new, Vec<Msg>) carries sync.prefs_too_large (new code). It is not an error and does not count toward unreachable.
• Once the sync data's preferences fit again and this device's did not change meanwhile, they land as synced. They do not count as a local edit and do not cause a conflict.
• The control plane's apply also skips oversized preferences instead of failing, as a guard.

twcore sync enable

"keep this device's version" (k) and "keep both" (r) now say that this device's version is uploaded with the join and replaces the item of that name on the other devices. For r, the other devices also get the renamed copy.

Tests

• tw-sync/tests/regressions.rs: the test host now stores preferences and refuses oversized ones the way the control plane does. The new test covers: the rest applies, the notice shows (also on the shortcut round), uploads keep the sync data's preferences, and smaller preferences land later without a conflict.
• tw-control/tests/sync_regressions.rs: through the endpoints, the state is idle, there is no error and no failing_since_ms, notices[0].code == "sync.prefs_too_large", and the rest is applied.
• Local: cargo test --workspace 3846 passed, 0 failed; cargo clippy --workspace --all-targets -D warnings clean; cargo fmt --check clean; scripts/smoke.sh 84 passed, 0 failed.

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