#496 Make best legend placement content-aware

Closes #485

What changed

The existing loc="best" pass only scored sampled point anchors once during payload construction. It could miss line crossings, area fills, marker extents, bar rectangles, and annotations, and the browser had no automatic-placement intent left to reconsider after layout or view changes.

This PR:

• measures the real legend footprint and scores the nine standard in-plot candidates against bounded emitted geometry for line, scatter, area, bar/column/histogram, density, and annotations;
• ships a concrete deterministic fallback plus auto_loc="best", so older clients and static output remain safe while the live renderer can refine placement;
• re-scores a fixed 96×72 rendered-pixel occupancy raster after responsive resize, settled pan/zoom, legend toggles, data updates, and LOD updates—never on every gesture frame and never by rescanning source rows;
• preserves explicit locations, anchors, and polar legend placement exactly;
• remeasures automatic legends against final SVG/raster export dimensions; and
• documents the placement, tie-break, wire, lifecycle, and bounded-work contracts.

Before / after

Same chart, data, 760×440 chart size, 800×500 Chrome viewport, and DPR 1. The baseline is commit 036271ac; both captures used their tree's own standalone client.

Before — legend obscures the rising line	After — legend moves to the empty corner

Capture provenance, DOM locations, dimensions, and SHA-256 hashes

Verification

• uv run --with pre-commit pre-commit run --all-files
• uv run ruff check .
• uv run ruff format --check .
• .venv/bin/ty check
• npm run typecheck
• npm run build
• full suite: 4,314 passed, 108 skipped
• focused real-Chromium adaptive-placement suite: 12 passed

Summary by CodeRabbit

• New Features

  • Added content-aware automatic placement for unanchored Cartesian legends using loc="best".
  • Legends avoid chart marks and annotations based on rendered geometry.
  • Placement updates after resizing, view changes, data updates, and responsive exports.
  • Added stable placement behavior with fallback when measurements are unavailable.
  • PNG and SVG exports recalculate placement when dimensions change.
  • Explicit locations, anchors, and polar legends remain unchanged.

• Documentation

  • Added guidance and examples for automatic legend placement and its behavior.