Implement reconcile from its specification

reconcile in src/ledgerlite/reconcile.py is declared, documented and

tested, but its body raises NotImplementedError. Write it.

The full contract is in the module's docstring: what a statement line is, what

a candidate is, the two passes (the window and the reference), how ties are

broken, what is unmatched, what is open, and what difference_cents

means. Read it before writing a line of code; it is the specification, and the

evaluation measures against it.

The short version

• A line claims exactly one candidate and a candidate is claimed by at most one

line; an amount must be exactly equal, sign included.

• Pass 1 looks only inside window_days; pass 2 is reference-driven and the

window does not apply.

• Lines are always considered oldest first; among candidates the closer, then

earlier, then smaller-id booking wins.

• open_candidates are entries that still carry an unclaimed candidate;

difference_cents is the sum of the amounts the statement could not place.

Boundaries

• Implement it in src/ledgerlite/reconcile.py; the public API of

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

• The function is pure: it reads the journal and the statement and returns a

Reconciliation; calling it twice on the same input returns the same answer.

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