fix: accept integer capsize in errorbar (#2020)

[krystophny]

Closes #2020.

Problem

matplotlib's idiomatic errorbar(..., capsize=4) (integer) failed to
compile in fortplot:

call errorbar(x, y, yerr=yerr, capsize=4)
                                        1
Error: There is no specific subroutine for the generic 'errorbar' at (1)

capsize was declared real(wp) only, so an integer literal matched no
specific procedure of the errorbar / add_errorbar generics. The reporter
read this as "errorbar does not support caps". (Caps render correctly when
capsize is passed as a real, e.g. capsize=4.0_wp.)

Fix

Add integer-capsize overloads for errorbar and add_errorbar (both the
RGB-triple and string color forms) as thin shims that convert to real and
forward to the existing specifics. The integer capsize argument is
required (not optional) in each new variant, which keeps it distinguishable
from its real-capsize counterpart and from the no-capsize call, so generic
resolution stays unambiguous.

Verification

Build (serial to avoid the unrelated fpm parallel-build SIGABRT):

OMP_NUM_THREADS=1 fpm build

Failing before the fix:

$ fpm build   # program calling errorbar(x, y, yerr=yerr, capsize=4)
Error: There is no specific subroutine for the generic 'errorbar'

Passing after the fix:

$ fpm test --target test_errorbar_capsize_integer
 PASS: integer capsize accepted and matches real capsize

The new test (test/plot_types/errorbar/test_errorbar_capsize_integer.f90)
asserts capsize=4 compiles and stores the same cap length (4.0) as
capsize=4.0_wp, read back via figure_t%get_plots().

Existing errorbar tests pass unchanged: test_errorbar_defaults,
test_errorbar_ascii_rendering, test_errorbar_pdf_rendering,
test_errorbar_legend_entry, test_errorbar_barsabove_warning,
test_stateful_api_fast. Rendering gate (make verify-artifacts) passes.
A manual render with capsize=4 shows the horizontal caps drawn at the
error-bar ends.