test: stabilize overlapping polar-area label fixture

[jakezwang]

The controller.polarArea/pointLabels/overlapping fixture fails on Ubuntu Firefox with a 293-pixel (0.11%) difference against its 0.1% tolerance. The same failure reproduces on pristine master and on the types-only #12321, with identical per-browser canvas output. This is the shared failure reported during #12321.

Disable image smoothing in this fixture's existing options.run callback before redrawing its sprite text, and regenerate only its expected PNG from Ubuntu Chrome. This removes browser-dependent interpolation of the overlapping glyph sprites at fractional coordinates. Captured point-label coordinates, chart area, and scale radius remain identical within each browser, including all 50 visible labels and the same 35 overlapping label pairs. The comparison threshold and tolerance remain unchanged.

Validation:

• Baseline reproduction: Firefox 155 and 156 both reproduce the unchanged fixture failure on pristine master and the PR.
• Final Ubuntu validation: complete Karma CI suite with --auto-watch --single-run --coverage --grep; 1703/1703 passed in each of Chrome 154, Firefox 155, and Firefox 156, plus the isolated fixture comparison in each browser. Every test command is required to return zero for this diagnostic run to pass.
• Changed fixture ESLint and git diff --check passed. Increasing point-label padding from 5 to 6 still fails the canonical image comparison (5.13% difference), verifying the fixture continues to detect layout changes.

AI assistance: implementation, testing, and this description were prepared with OpenAI Codex.