fix(render): adapt axis ticks to available plot space

[nmrtist]

Summary

Axis tick counts were previously fixed at eight for x and five for y,
regardless of the final plot dimensions. Because PlotX uses absolute
publication-sized typography, shrinking a plot could cause adjacent tick
labels to overlap.

This change introduces a shared, size-aware axis layout used by the screen,
SVG, and EMF renderers. Tick density is derived from the final plot rectangle
after margins, projection bands, and aspect-ratio locking are applied.

Key changes:

• Move shared tick generation, formatting, and width estimation into a
  dedicated render module.
• Add AxisLayout, which computes margins and x/y tick sets together for the
  requested output dimensions.
• Thin numeric and categorical ticks according to available width and height.
• Validate actual x-label intervals after nice-tick generation rather than
  assuming the requested target equals the resulting tick count.
• Drop an axis's ticks and labels when two labels cannot fit, while preserving
  the axis line.
• Keep canvas zoom independent from layout density by calculating screen
  layouts in page units.
• Use the same adaptive margins for painting, export, hit testing, and phase
  editing so rendered and interactive plot rectangles stay aligned.
• Make the first layout pass conservative across every visible category,
  preventing later stride choices from increasing margins and invalidating
  label-fit checks.
• Reserve space for endpoint x labels even when y ticks are dropped.
• Use Unicode East Asian Width data for CJK, full-width, supplementary-plane,
  combining, and other Unicode characters.
• Escape categorical tick labels in SVG output.
• Clarify the intrinsic-size semantics of Margins::for_figure.
• Update the English and Simplified Chinese layout documentation.

The intentionally conservative categorical first pass may reserve space for a
long category that is not ultimately labeled. This tradeoff ensures the final
plot rectangle never becomes smaller than the rectangle used to validate tick
spacing.

Validation

• cargo test -p plotx-render — 48 tests passed
• cargo pr-check — all 7 checks passed, including:
  • formatting
  • Rust source-size limits
  • dependency policy
  • default frontend builds
  • Clippy with warnings denied
  • all-feature workspace tests
  • reference-backend tests
• npm run build from docs/ — passed
• git diff --check — passed

Regression coverage includes:

• adjacent x-label interval separation across narrow and wide figures
• monotonic tick-density changes
• extremely small figures retaining their axis lines
• long categorical labels and changing categorical strides
• CJK and supplementary-plane character widths
• reversed axes and scientific multipliers
• aspect locking and marginal projection bands
• SVG escaping for user-provided category names
• screen/export layout consistency