Fix Cartesian text clipping by iterative range-space extent convergence in _finalize()

[eaton-lab]

Problem

Cartesian._finalize() had two related issues:

1. Large text near plot boundaries could be clipped because range-space extent fitting was solved in a single pass.
2. In some scenarios, overflow computation could attempt numpy.max() on empty extent arrays, raising:
   ValueError: zero-size array to reduction operation maximum which has no identity.

What this PR changes

• Refactors Cartesian finalization into:
  • _finalize_once() (original single-pass finalize logic),
  • _finalize() wrapper that iteratively refines extent fitting.
• Uses screen-space overflow as convergence criterion.
• Adds private convergence bounds:
  • max_iter = 6
  • px_tol = 0.05
• Corrects y-axis overflow orientation handling in screen coordinates:
  • _ymin_range is top, _ymax_range is bottom.
• Adds safe overflow reduction logic for empty extent arrays (no zero-size numpy.max()).
• Keeps unreachable-fit behavior bounded: when text cannot fit in drawable range, iteration stops at max_iter and allows remaining overflow.

Why this is safe

• No public API changes.
• Behavior changes are internal to Cartesian finalization.
• Existing rendering semantics are preserved outside bug-fix behavior.

Convergence constants (private)

• _CARTESIAN_FINALIZE_MAX_ITER = 6
• _CARTESIAN_FINALIZE_PX_TOL = 0.05

Tests

Adds regression coverage in features/cartesian-coordinates.feature and corresponding steps:

• Unreachable text extent uses bounded fallback iterations
  • constructs an intentionally unreachable-fit text case,
  • asserts finalize passes are bounded (<= max_iter),
  • asserts remaining overflow is still above tolerance (expected).

This codifies expected fallback behavior for geometrically impossible fit cases.

Validation

• Clipping repro for large boundary text is resolved by iterative finalize.
• Prior mass-error path from empty extent reductions is removed.
• Cartesian rendering scenarios no longer fail due to zero-size overflow reductions.

Commits

• d5023b79 Add regression scenario for unreachable cartesian text extents
• 7dfb935f Fix cartesian finalize text extent convergence and overflow bounds

Example

import toyplot
import toyplot.html
import numpy as np

rng = np.random.default_rng(123)
canvas = toyplot.canvas.Canvas(width=400, height=400)
axes = canvas.cartesian()
axes.scatterplot(rng.random(100), rng.random(100))
mark = axes.text(
    5,
    5,
    "hello world",
    color="black",
    style={"text-anchor": "start", "font-size": 28},
    annotation=False,
)
toyplot.html.render(canvas)

(xdat, _), (_, right, _, _) = mark.extents(["x", "y"])
overflow = float((axes.project("x", xdat) + right)[0] - axes._xmax_range)
print("right-overflow-px:", overflow)
# right-overflow-px: 0.04236587863653085
canvas

[eaton-lab]

Note: all tests passed when run locally.

[coveralls]

coverage: 94.148% (+0.03%) from 94.119%
when pulling dc2ef04 on eaton-lab:cartesian-iter-finalize-clip
into f71796a on sandialabs:main.

[eaton-lab]

I believe these errors are unrelated to this PR. I raised separate PRs for them:

#226 ReportLab / Python 3.8 compatibility issue.

#225 : numpy v1/2 compatibility error during toyplot.graph

[tshead2]

This looks like a winner, thanks for your contribution!