Summary

Adds a Stacker capability for sequential / LIFO plate storage - the code counterpart to the "Stacking Access (Sequential)" category that's already described in the storage user guide but had no implementation (today every storage device is forced through the random-access Incubator).

Design per the discussion in #1113. @rickwierenga preferred a composable capability over a split-frontend hierarchy, with the transfer position as part of the stacker, so this:

• is a standalone capability wrapping a StackerBackend (no shared StorageBackend base, no changes to Incubator);
• owns the transfer position (the "loading tray", borrowing the incubator's term).

What's here

• Stacker(Machine, Resource) - holds one or more single-ended LIFO stacks (each a ResourceStack(direction="z")) plus a loading tray.
  • downstack(stack) → moves the accessible (top) plate onto the loading tray and returns it.
  • upstack(stack, plate=None) → moves a plate from the loading tray onto a stack (defaults to whatever is on the tray).
  • LIFO is enforced by ResourceStack (only the top plate is accessible); stack height/positions come from Plate.stacking_z_height via the nesting added in ResourceStack: nest stacked plates by Plate.stacking_z_height #1112.
  • Helpers: get_accessible_plate, get_stack_by_plate_name, summary.
• StackerBackend — minimal device interface: set_stacks, downstack, upstack. Deliberately no door/temperature/shaking (a stacker is not an incubator).
• StackerChatterboxBackend — no-op backend for tests/demos.
• 10 tests (LIFO order, nesting height, empty-stack / occupied-tray / no-plate errors, foreign-stack rejection, etc.).

Intended consumers

The Agilent BenchCel (#1109) and the HighRes MicroServe. Migrating the BenchCel from its current faked-PlateCarrier Incubator model onto this capability is a planned follow-up.

Notes / follow-ups

• Serialization round-trip is intentionally not implemented yet — it needs ResourceStack serialization support (ResourceStack.__init__ takes direction, not size_*, so it doesn't round-trip through the generic path). Stacker.serialize works; Stacker.deserialize raises NotImplementedError for now (and resolves the Machine/Resource MRO ambiguity).
• Open design questions from Sequential/stacking ('Stacking Access') storage abstraction backed by ResourceStack #1113 still worth your call: a generic "transfer position" abstraction shared with incubators, and whether carousels fold in later.