define Geom class hierarchy skeleton (no behavior change)

[AviraL0013]

Overview

This PR initiates the refactoring of the monolithic draw_geom() function in inst/htmljs/animint.js into a structured, object-oriented Geom class hierarchy.

This work builds upon the foundation started by @Faye-yufan in PR #107, I am restoring those architectural decisions and extending them to cover missing geoms (GeomTallrect, GeomWiderect, GeomRect).

Architectural Changes

I have introduced a base Geom class and two virtual classes to handle the core data-binding distinction in animint2:

1. Geom (Base): Handles shared logic for panel identification, SVG selection, and scale lookup.
2. GroupGeom (Virtual): Designed for geoms like path, line, and polygon (data_is_object: true). These geoms require the "kv-based" data-bind trick to bind one SVG element to a group of data rows.
3. UngroupGeom (Virtual): Designed for standard geoms like point and segment. These bind data row-by-row (one SVG element per data point).

Concrete Subclasses Implemented (Skeleton):

• GeomSegment, GeomLinerange, GeomVline, GeomHline, GeomText, GeomPoint (Restored from Faye's design)
• GeomTallrect, GeomWiderect, GeomRect (New additions)

Purpose and Future Work

This refactor is a critical architectural prerequisite for:

• Issue Move scale range calculation from compiler to the renderer. #48: Moving scale range calculation to the renderer. Each class will implement a get_domain() method to return its visible data range.
• Polygon Holes: Cleaner implementation of d3.geoPath within a dedicated GeomPolygon class.

[AviraL0013]

Hi @Faye-yufan @tdhock! Started fresh from master as suggested. Pushed the initial skeleton with Geom -> GroupGeom/UngroupGeom split and concrete subclasses for all ungrouped geoms (GeomTallrect, GeomWiderect, GeomRect included). Will migrate geoms one-by-one let me know if the direction looks right before I go further.