test(shape): cover move_vertex and translate mutation methods

[wkentaro]

Shape.insert_point and remove_point are unit-tested, but their sibling
mutation methods move_vertex (drag a vertex) and translate (drag a whole
shape) had no direct coverage despite being core editing primitives. These tests
close that gap: they pin sub-pixel float precision, assert move_vertex changes
only the target vertex, and assert translate shifts every point by the offset
without changing the point count.

Test plan

• uv run pytest tests/unit/_shape_test.py -q (49 passed)
• uv run ruff check / uv run ty check on the changed file

[wkentaro]

This was generated by AI during PR processing.

Verdict: recommend-merge

Test-only PR adding coverage for Shape.move_vertex and Shape.translate. Reviewed:

• move_vertex(i=1, ...) correctly asserts only points[1] changes and the other three vertices are untouched.
• translate(offset=...) correctly asserts all four vertices shift by the offset (verified arithmetic against the (0,0)-(10,10) square).
• Both tests pass locally; names, kwargs, the _make_square_polygon helper, and pytest.approx usage match repo conventions.

CI is green across all 8 matrix jobs; branch is current with main (0 commits behind) and MERGEABLE. No findings.