TikZ backend: coordinate scaling policy (display units → cm)

[yueswater]

Sub-issue of #50.

Problem

Matplotlib delivers display coordinates (points / pixels). TikZ works
natively in cm. Without a conversion factor, a typical 6×4 in figure
explodes into a 15+ cm picture that breaks page layout in textbooks and
slide templates.

Proposed fix

Expose scale as a constructor argument on TikzRenderer (default 0.5,
cm per display unit after Matplotlib's internal DPI) and surface an
override at the Canvas.save(..., tikz_scale=...) layer.

Optionally derive a default from fig.get_size_inches() so a 6×4 in
figure maps to a reasonable ~7.5×5 cm TikZ picture without manual tuning.

Acceptance criteria

• TikzRenderer(..., scale=s) multiplies every emitted coordinate by s.
• Canvas.save('plot.tex', tikz_scale=0.4) propagates cleanly.
• A Cobb–Douglas example renders within a target bounding box
  (e.g. ≤ 10 cm wide) by default.
• Scale behavior documented alongside other Canvas.save options.