CrossSection: drop FillRule/Compose, add SetTolerance/GetTolerance

[zmerlynn]

Summary

Part of #1707. Breaks the CrossSection public API now that the boolean2 backend
is the only backend (#1751).

Dropped:

• FillRule enum and constructors that accepted it. The boolean2 backend
  supports only positive-winding fill; the enum had no effect since Cut over CrossSection from Clipper2 to boolean2 #1751, and
  passing any non-Positive value triggered a DEBUG_ASSERT in assert-enabled
  builds. Callers passing FillRule::Positive (the only valid value) just drop
  the argument.
• Compose(const std::vector<CrossSection>&) static. It was a batch boolean
  union - identical to BatchBoolean(..., OpType::Add). Use BatchBoolean or
  the + operator instead.
• ManifoldFillRule enum from the C header (types.h).

Added:

• GetTolerance() / SetTolerance(double) - mirrors Manifold::GetTolerance /
  Manifold::SetTolerance. GetTolerance returns the cross-section's propagated
  drift budget. SetTolerance returns a copy with it adjusted: raising simplifies
  to the new budget; lowering floors at the geometry's natural epsilon.

Changed:

• Simplify(double epsilon = 1e-6) -> Simplify(double tolerance = 0). Default
  0 uses the section's own tolerance (from GetTolerance()). Callers passing
  an explicit non-zero value are unaffected.
• Hull(SimplePolygon pts) / Hull(Polygons polys) now take const-ref.
  HullImpl sorts in place; the old by-value signature obscured this.
• Rect, Square, and Circle constructors now set tolerance_ from
  InferEps on the output geometry, consistent with the Polygons constructor.
  Previously GetTolerance() returned 0 for these paths.

Fixed (pre-existing):

• WASM JoinType: 'Bevel' was missing from the TypeScript type and from
  joinTypeToInt in bindings.js, so passing 'Bevel' silently fell through
  to 'Square'.

C, Python, and WASM bindings updated throughout.

Test plan

• 119 tests pass (58 CrossSection + 39 Boolean2 + 22 CBIND existing +
  1 new CBIND.cross_section_tolerance). 2 seed-queue CrossSection tests remain
  disabled pending fixes on a separate branch.

[codecov]

Codecov Report

❌ Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.83%. Comparing base (ecc145f) to head (dd078c1).

Files with missing lines	Patch %	Lines
src/cross_section.cpp	95.83%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1760   +/-   ##
=======================================
  Coverage   93.83%   93.83%           
=======================================
  Files          44       44           
  Lines        9985     9992    +7     
=======================================
+ Hits         9369     9376    +7     
  Misses        616      616           

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[elalish]

Thanks, looking much cleaner!