Accept raw allocation sequences and streamline the public API

[fpedd]

Let every public entry point (allocate, validate_allocation,
save_allocation, plot_allocation) accept a raw Sequence[Allocation] in
addition to System/Memory/Pool, wrapped via the new
Pool.from_allocations; allocate returns the placed allocations as a
tuple in input order. Pool.allocate now restores the pool's input order
after allocator-internal reordering so positions keep corresponding to
the request, and checks the returned set by length as well as ids.

Rename Memory.capacity to Memory.size (validation message follows), and
rename the analysis functions pressure/pressure_per_allocation to
antichain_pressure/antichain_pressure_per_allocation, keeping the old
names exported as aliases for the canonical default metric.

Drop the keyword-only markers from allocator constructors, Timer, and
plot_allocation so the common single-argument calls read naturally. Add
any_allocated across Pool/Memory/System and use it in io. Add nanobind
to the dev dependency group.