Fix audit bugs (#1, #2, #3) and regenerate distorted logo

[milkway]

Addresses the package audit findings.

Bug fixes

#1 — Memory leak in pikchr_c() (src/pikchr.c)

pikchr() returns a malloc/realloc'd buffer that was never freed, leaking the entire SVG string on every render. Now free()d after copying into the R string. Also guards a possible NULL return (mkChar(NULL) would crash R) by emitting NA, and replaces the legacy STRING_VALUE macro with CHAR(STRING_ELT(...)).

#2 — Broken height default (R/pikchrWrappers.R)

if (is.null(width)) width <- bbox_width
if (is.null(width)) width <- bbox_height   # tested `width` again -> always FALSE

The second guard now tests height, so it correctly defaults to the viewBox height instead of producing an empty height:; CSS rule.

#3 — Documented defaults didn't match actual defaults (R/pikchrWrappers.R, man/pikchr.Rd)

Aligned @param docs with the real signature: width/height → viewBox intrinsic, fontSize → 100%, fontFamily → Jost, align → none. Docs regenerated with roxygen2.

Logo

Regenerated man/figures/pikchrHex.svg and man/figures/logo.png with a geometrically regular hexagon (was squished, width/height 0.73 vs 0.866) and fixed the invalid <def> element (→ <defs>). Deterministic generator added under tools/ (excluded from the build via .Rbuildignore).

Verification

• R CMD SHLIB pikchr.c init.c compiles cleanly (no warnings).
• Functional check: valid diagram returns <svg>; error path returns a non-NA character scalar.

Closes #1
Closes #2
Closes #3

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