fix: PDF pie chart legend box overlaps slice label (fixes #1680)

[krystophny]

Summary

The 'east' legend location string was unrecognized by legend_set_position and fell through to LEGEND_UPPER_RIGHT, placing the legend inside the plot area where it overlapped pie slice labels in PDF output. The merged text appeared as "OnlineNorth" instead of two separate entries.

Added LEGEND_EAST position constant that places the legend outside the plot area to the right, centered vertically — matching matplotlib semantics.

Changes

• src/ui/fortplot_legend.f90: new LEGEND_EAST constant (5); 'east' case in legend_set_position; ASCII layout case
• src/ui/fortplot_legend_layout.f90: LEGEND_EAST handling in calculate_legend_position
• src/backends/ascii/fortplot_ascii.f90: LEGEND_EAST case in ascii_calculate_legend_position
• src/backends/ascii/fortplot_ascii_legend.f90: LEGEND_EAST case in calculate_ascii_legend_position
• test/test_pie_legend_east_pdf.f90: regression test — creates pie chart with 'east' legend, saves PDF, verifies no merged text

Verification

Test passes after fix

$ fpm test --target test_pie_legend_east_pdf
 PASS: pie legend at east position does not merge with slice labels in PDF

CI test suite passes

$ make test-ci
CI essential test suite completed successfully

Artifact verification passes

$ make verify-artifacts
Artifact verification passed.

PDF text extraction confirms fix

Before: legend text and slice label merged into single string in PDF.
After: pdftotext shows "Online" and "North" as separate text entries:

$ pdftotext stateful_sales.pdf -
Regional revenue share
Online
North
15.0%
30.0%
...

[krystophny]

Review: Issue-Resolution Verification

Issue #1680: PDF pie chart legend box overlaps slice label, first entry label truncated ("OnlineNorth")

Requirement	Evidence
`"east"` location string unrecognized	FIXED: `legend_set_position` handles `"east"` at `fortplot_legend.f90:123-124`
Legend fell through to `LEGEND_UPPER_RIGHT` causing overlap	FIXED: `LEGEND_EAST` (5) places legend outside plot area at `fortplot_legend_layout.f90:247-249`
PDF merged text ("OnlineNorth")	FIXED: regression test `test_pie_legend_east_pdf.f90` verifies separate `"(North)"` and `"(Online)"` in PDF stream
PNG unaffected	Confirmed: fix only affects `"east"` path; default behavior unchanged
ASCII backend coverage	FIXED: `fortplot_ascii.f90` and `fortplot_ascii_legend.f90` both handle `LEGEND_EAST`
No scope creep	Confirmed: only `LEGEND_EAST` constant + `"east"` case added; trailing whitespace cleaned

Code Review

• `fortplot_legend.f90`: constant, public export, `select case` — correct
• `fortplot_legend_layout.f90`: `box%x = data_width + margins(1)` places legend outside plot area; `box%y` centers vertically — correct matplotlib semantics
• ASCII backends: positioning consistent with `LEGEND_EAST` semantics
• Test: PDF stream text extraction with explicit merged-text negative check — well-designed regression test
• Module sizes within limits (`fortplot_legend.f90` at 600 lines < 1000 hard limit)

Deferred (polish, tracked separately)

• SVG backend (`fortplot_svg.f90:536-558`) lacks `"east"` case — falls to default. Tracked in polish: add 'east' legend position support to SVG backend #1826.

Verdict: PR fully resolves issue #1680. All requirements addressed with evidence. No blocking issues. Ready for merge after CI completes.