Implement allocate from its specification

allocate in src/stockpile/allocate.py is declared, documented and tested,

but its body raises NotImplementedError. Write it.

The full contract is in the module's docstring: which lots may give, how a

lot that has already been promised to an earlier plan is treated, the FEFO

order and its two tie-breaks, how much is taken from each lot, and why a

partial plan is never returned. Read it before writing a line of code; it is

the specification, and the evaluation measures against it.

The short version

• A lot may give when it is in the warehouse, not expired at now — a lot that

expires on now is still eligible — and still has free quantity, counting

whatever already_planned has promised from it.

• Earliest expiry first; ties go to the oldest receipt, then the smaller lot

id. The earliest eligible lot is drained as far as it can go, then the next.

• If the eligible lots cannot cover the request, refuse the whole request with

InsufficientStock; a plan that ships half its lines is worse than one that

is refused up front.

• The result is a plan, not a movement: picks add up to the request and

to_reservation turns the plan into something Stock.reserve can hold.

Boundaries

• Implement it in src/stockpile/allocate.py; the public API of stockpile

stays as it is, and nothing else in the package changes.

• The function is pure: it reads the stock and returns an Allocation;

calling it twice on the same input returns the same plan.

• Do not add third-party dependencies — the standard library is enough.
• The repository's own test suite (pytest) has to stay green.