Label ProportionShift top bars with system names (#47)

[andyreagan]

Fixes #47.

Problem

ProportionShift.get_shift_graph leaves the cumulative bars at the top of the plot unlabeled, unlike the JSD shift graph where each system's name is written next to its purple/orange bar. As the issue reporter found, adding all_pos_contributions=True (the difference from the JSD definition) does add the names but breaks the plot by pushing every bar to the right-hand side.

Cause

all_pos_contributions conflates two things in the plotting layer:

1. Bar direction — in get_bar_dims, it flips a type's bar based on the sign of p_diff. This is correct for JSD, whose contributions are inherently non-negative. But proportion shift scores already carry their sign (positive when a type is more prevalent in system 2, negative for system 1), so flipping forces every bar positive → all bars point right.
2. Top-bar labels — only the all_pos_* bar order uses the system names as symbols; the neg_total/pos_total bars used by ProportionShift have empty symbols.

So the reporter's workaround couldn't win: getting the labels also flipped the bars.

Fix

Separate the two concerns. Add a label_total_with_system_names plot param that labels the neg_total/pos_total bars with the system names without changing bar direction, and enable it for ProportionShift. Those bars already share the same purple/orange colors as the JSD all_pos_* bars, so the result visually matches the JSD graph.

• shifterator/plotting.py: new label_total_with_system_names param (default False); when set, the neg_total/pos_total symbols become the system names.
• shifterator/shifts.py: ProportionShift.get_shift_graph opts in via kwargs.setdefault(...) so callers can still override.

Tests

Added TestIssue47ProportionShiftSystemNames in tests/test_issues.py:

• system names appear on the graph (custom and default names)
• bars remain two-sided (regression guard against the all-right-side breakage)
• other shift types are unaffected

Full suite: 80 passed locally (uv run pytest).

Note

This PR also bumps the version to 0.5.1 and exposes shifterator.__version__, so a branch install can be confirmed with import shifterator; print(shifterator.__version__).