fix(plot): prevent crashes, hangs, and wrong-series rendering

[balloob]

Summary

Correctness and robustness fixes for the plot and diagram elements. These address pathological inputs that can crash the render or block the Home Assistant event loop, plus a couple of wrong-output bugs. Each fix is small and self-contained.

Findings addressed

• A1 — wrong series style. draw_plot skipped entities whose states were all invalid when building raw_data, then zipped raw_data against the full element["data"] config list. An unavailable first entity would shift the mapping, so a later series rendered with the wrong entity's color/width/smooth. Segments are now paired with their own config.
• A2 — event-loop hang (DoS). The y-axis tick/grid/legend loops step from min_v to max_v by tick_every (default 1). One bogus sensor reading (a counter, a glitch to 2**32) makes the spread millions wide and each loop issues millions of draw calls, freezing the async loop. Tick spacing is now clamped so at most _MAX_Y_TICKS (100) ticks are drawn; reasonable configs are untouched.
• A3 — ZeroDivisionError in diagram. The all-bars-zero guard set ctx.pos_y but was missing a return, so execution fell through to height_factor = (height - offset_lines) / max_val. Added the return.
• A5 — 3-point smooth dropped the middle point. The Catmull-Rom boundary extension only ran for len(points) > 3, so a 3-point series collapsed to a straight first→last line (the peak vanished). Boundary extension now runs at >= 3 points.
• A10 — documentation. Clarified in the docstring that low/high seed but do not pin the y-axis range (data still auto-expands it).
• B6 — dotted grid perf. Dotted grid lines called draw.point() once per pixel; now batched into a single draw.point(seq) call per line.
• B8 — smoothing perf. The four Catmull-Rom basis weights depend only on the step parameter and are constant across segments; they're now precomputed once per smoothing call instead of per interpolated point.

Tests

Adds regression tests for A1 (skipped-entity style leakage), A2 (huge spread renders without hanging), A3 (all-zero bars), and A5 (3-point smoothing keeps the middle). Full suite: no new failures (the pre-existing tests/visual/* byte-snapshot failures under Pillow 12 are unrelated and addressed separately).

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 96.66667% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/odl_renderer/elements/visualizations.py	96.66%	1 Missing ⚠️

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