fix(saveslot): carry accrued playtime onto a copied slot

[buildthomas]

Problem

Copying a save slot loses its playtime — the copy shows as 0 time played even though it carries the same progress.

Both copy paths seed the new slot with only SlotName and Summary:

• HasSaveSlots.PromiseDuplicateSlot passes those two to PromiseCreateSlot
• PromiseExportSlot builds a SaveSlotExport of {data, slotName, summary}, and PromiseImportSlot seeds the new slot from those

TimePlayed is never part of that hand-off, so it starts nil on every copy.

Fix

Playtime travels with the progress it measures:

• SaveSlotCreateMetadata takes an optional TimePlayed, applied in PromiseCreateSlot (it is already one of MUTABLE_METADATA_KEYS, so _buildSlot persists it like any other metadata field)
• PromiseDuplicateSlot and PromiseImportSlot pass the source's value through
• SaveSlotExport gains timePlayed, written by PromiseExportSlot and validated as an optional number by isSaveSlotExport
• Both duplicate and export call _flushPlaytime() first, so copying the active slot includes the running session rather than only what the last save wrote

Timestamps and session counters (PlayCount, LastSessionLength) still start fresh on a copy — those describe the new slot's own sessions, not the progress being copied.

Backwards compatibility

• Exports written before this change have no timePlayed; they still validate and import to a slot with no accrued playtime, exactly as they did when written
• A newer export read by older code just carries an unused field
• No change to how slot metadata is stored or loaded: TimePlayed was already an optional attribute, this only seeds it at creation
• SaveSlotExportUtils.create gained a trailing optional parameter, so existing 3-argument callers are unaffected

TimePlayed in the create metadata is not a new trust surface: PromiseSetSlotMetadata is already client-callable and accepts the full SaveSlotMetadata, TimePlayed included.

Tests

New coverage in HasSaveSlots.spec.lua (duplicate carries playtime including the unflushed live session; a never-played slot's copy still has none), HasSaveSlots.ExportImport.spec.lua (export/import round-trips playtime, exports the live session, imports a pre-change export, refuses a non-numeric timePlayed), and SaveSlotExportUtils.spec.lua (create/validation).

stylua and selene pass. I could not run nevermore test --cloud here — no Open Cloud key in this environment, and the local rojo shim is broken on this machine — so the new specs have not been executed against the engine.