Adds the InvoiceLineNumbering class that InvoiceDraftCalculator.
RecomputePositions, InvoiceSetPricing.AssignPositionLabels and their
callers already depend on (this dependency was pushed in ecc74cf
without the class itself, breaking the build) — position labels are
now hierarchical ("1", "1.1", "1.2", …) matching a line's ADR 0016
nesting level, computed by one pure, shared algorithm for both the
online editor and the PDF so they can never disagree (ADR 0008).
Also fixes an InvoiceDraftServiceTests assertion that still expected
the old flat integer position field; it is a string label now.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
