Add symbol size/shape translator for tmap and mapgl

[NewGraphEnvironment]

Problem

The gq registry stores point mark properties as radius (in QGIS mm units, e.g. 2.0, 2.4, 3.0), but gq_tmap_style() only returns fill and size for points — and the size value from the registry doesn't translate well to tmap's size units. A registry radius of 2.4 becomes nearly invisible in tmap at default figure dimensions.

This means every map manually guesses tmap size values through trial and error. The same problem will exist for mapgl circle radius.

Proposed Solution

Add a symbol size translator that converts registry mark radius to target-appropriate sizes:

# Returns size calibrated for tmap
gq_tmap_style(reg$layers$fiss_obstacles)
# $fill = "black", $size = 0.5  (not 2.0)

# Or a standalone converter
gq_symbol_size(radius = 2.4, target = "tmap")  # returns 0.5
gq_symbol_size(radius = 2.4, target = "mapgl")  # returns 6

The conversion factor depends on the target renderer:

• tmap v4: sizes in proportional units (0-1 range typical for maps)
• mapgl: circle-radius in pixels
• QGIS: mm (already in registry)

Could also handle shape mapping — QGIS shape names (square, triangle, circle) to R pch codes or mapgl equivalents.

Discovered while building the fresh subbasin vignette — falls (registry radius 2.0) and fish observations (radius 2.4) were invisible at their registry sizes in tmap.

Relates to #9