fix(task-17): an inch sheet's fit reports paper units per drawing unit, not the mm ratio times 25.4

`_paper_frame` computed `paper_per_unit = mm_per_unit * unit` where `unit`
is millimetres per paper unit, so an extents+fit plot on an inch sheet
reported `effective_scale: "607.06:1"` for a fit that is `1:1.06276` (off
by 25.4 squared). The render was right (`units_per_mm` is unit-free); only
the label was wrong, and it flowed into `drawing_export_pdf(layout=...)`.
The mm sheet could not see it because there `unit` is 1.

`test_extents_plot_fits_the_printable_area_and_centres` is parametrized
over ISO A3 (mm, width-bound) and ANSI B (inches, height-bound); the ink
position is derived from the centred frame so both cases check the same
invariant. The inch case fails without the fix.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd

backends/ezdxf_backend.py

@@ -2524,7 +2524,12 @@ class EzdxfBackend(AutoCADBackend):
                 usable = [r for r in ratios if r is not None]
                 mm_per_unit = min(usable) if usable else 1.0
                 units_per_mm = 1.0 / mm_per_unit
-                paper_per_unit = mm_per_unit * unit
+                # ``unit`` is millimetres per paper unit (25.4 under inches),
+                # so paper units per drawing unit *divides* by it. Multiplying
+                # reported a 1:1.06 inch fit as ``607.06:1`` (off by 25.4^2)
+                # while the render itself was right, ``units_per_mm`` being
+                # unit-free; the mm sheet hid it because there ``unit`` is 1.
+                paper_per_unit = mm_per_unit / unit
                 effective = (
                     scale_label(paper_per_unit, 1.0)
                     if paper_per_unit >= 1.0