Migrate label geoms to the ggfittext fit_text_grob() constructor

[wilkox]

Goal

Migrate the label geoms off ggfittext's private grob internals onto its
public fit_text_grob() constructor (exported in ggfittext 0.11.0 —
wilkox/ggfittext#70).

Why this is a gate, not just cleanup

gggenes currently hand-rolls ggfittext's private grob. In R/compose_grob.R
the text branch constructs the ggfittext gTree directly:

grid::gTree(..., cl = "fittexttree")        # cartesian / flip
grid::gTree(..., cl = "fittexttreepolar")   # polar

This reaches into exactly the private contract that ggfittext 0.12.0 will
change (0.12.0 carries the coord_radial() polar-label fix and, per its
milestone, "changes the (private) linear gTree contract, so lands after the
downstream-migration gate"). ggfittext 0.11.0 exists specifically as that gate:
downstream packages migrate onto fit_text_grob() before the internals move.

Consequently gggenes must migrate to fit_text_grob() before it can adopt
ggfittext 0.12.0 — i.e. before coord_radial() (#96) can be finalised.
Skipping this would mean the ggfittext bump that unblocks coord_radial
simultaneously breaks existing label rendering.

Scope

• Replace both hand-rolled gTree constructions in R/compose_grob.R (the
  fittexttree and fittexttreepolar cases) with ggfittext::fit_text_grob().
• Bump DESCRIPTION to Imports: ggfittext (>= 0.11.0) (coordinate with the
  ggplot2 floor review, Review and rationalise the declared ggplot2 version floor (DESCRIPTION says >= 2.2.1) #107).
• Verify label rendering is unchanged (regenerate/inspect the vdiffr snapshots
  for every *_label geom across all coordinate systems).
• Confirm the new build/draw smoke tests still pass.

Dependencies

• Depends on: ggfittext#70 (export fit_text_grob()) landing on CRAN.
• Blocks: Support coord_radial() for circular genome visualizations #96 coord_radial(), which requires ggfittext ≥ 0.12.0 and hence
  this migration first.