vjust arg for gridify Object

[Polkas]

Adds a vjust slot to gridifyObject() so users can vertically anchor the rendered object inside its layout row (e.g. vjust = 1 pins a gt/flextable to the top of a taller row instead of stretching it).

New vjust argument on gridifyObject() (default 0.5; fully backwards-compatible).
print() now sizes the object's viewport to grobHeight() for fixed-size grobs when vjust != 0.5, floored at 1 inch via grid::unit.pmax(). Flexible grobs (ggplotGrob-style gtables with null heights, recorded gTrees with childrenvp) keep the historical "fill the row" behaviour.
Detection & expression building factored into is_flexible_grob() and object_viewport_height_expr() in gridify-utils.R, with unit tests.
S4 validity rejects NA, NaN, ±Inf, non-scalar and out-of-range values.
Roxygen on vjust / object_vjust (in simple_layout(), complex_layout(), pharma_layout_*) makes the flexible-grob caveat explicit; show_spec() annotates the Vjust line accordingly.
No changes for users who don't set vjust.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.68%. Comparing base (45855f1) to head (b32ca4b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #18      +/-   ##
==========================================
+ Coverage   99.56%   99.68%   +0.12%     
==========================================
  Files           9        9              
  Lines         919      953      +34     
==========================================
+ Hits          915      950      +35     
+ Misses          4        3       -1     

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