fix: route hist/bar/scatter to active subplot (#2021)

[krystophny]

Closes #2021.

Problem

Switching a plotting call from plot(x, y) to hist, bar, barh, or
scatter inside a subplot grid produced blank white axes: the figure
compiled and ran without error, but no data appeared.

Root cause: plot() routes into the selected subplot via subplot_plot,
while hist/bar/scatter build their plot_data into the figure-level
plots array. When a subplot grid is active the renderer draws the subplot
cells and never touches the figure-level array, so that data silently
vanishes.

Fix

Add figure_t%relocate_last_plot_to_subplot, which mirrors the most
recently added figure-level plot into the active subplot, and call it from
the hist, bar/barh (bar_impl/barh_impl), and scatter
(scatter_2d_dispatch) chokepoints. The figure-level slot is retained
rather than removed because the add routines assume monotonically growing
slots; the figure-level array is inert while a subplot grid is active.

Verification

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

OMP_NUM_THREADS=1 fpm build

Regression test (extends test/state/test_stateful_subplots_routing.f90)

Failing before the fix:

$ fpm test --target test_stateful_subplots_routing
 Running stateful subplot routing tests...
   FAIL: hist not routed to subplot (1,1)
STOP 1

Passing after the fix:

$ fpm test --target test_stateful_subplots_routing
 Running stateful subplot routing tests...
 All stateful subplot routing tests PASSED!

The test asserts hist, bar, and scatter each land in the selected
subplot (subplot_plot_count == 1) and do not leak into the others.

Rendering gate

$ OMP_NUM_THREADS=1 make verify-artifacts
...
[png] subplots_grid_demo.png size=44465
Artifact verification passed.

Reproduced the issue's 2x2 grid (hist / bar / scatter / plot) to
/tmp/repro_2021.pdf. pdftotext -layout shows every cell now carries
data-derived tick ranges instead of the blank 0.0-1.0 default:

hist  (top-left):    x 1.0..5.0,  y 0..4
bar   (top-right):   x 0..20,     y 0..5
scatter (bottom-left): x 2.5..20, y 1..5
plot  (bottom-right):  x 2.5..20, y 1..5

Full test suite passes (OMP_NUM_THREADS=1 fpm test; no test reports FAIL).