Reduce Rd bloat from deep R6 inheritance chains

[kapelner]

In a package with a deep R6 inheritance chain (subclass -> parent -> grandparent -> ...), every class's Rd page rendered a single flat "Inherited methods" list mixing in every ancestor's methods together. Since each level down the chain accumulates the methods of all its ancestors, this list grows with total inherited method count across the whole chain -- for a package with many R6 classes and inheritance depths of 5+ levels, this produced Rd files with hundreds of lines per class just for this one section, and (more importantly) made every one of those classes' pages large enough that repeated Rd-to-text conversion during R CMD INSTALL compounded a separate, unrelated R-core memory issue we ran into and reported upstream.

This adds a new r6_inherited_documentation_display option (settable via Config/roxygen2/r6_inherited_documentation_display in DESCRIPTION, or man/roxygen/meta.R) with three values:

• "grouped" (new default): one collapsed subsection per ancestor class that actually contributed an inherited method ("+ inherited public methods from "), nearest ancestor first, each still listing its own methods as linked bullets -- same information as before, but organized by ancestor instead of flattened, and collapsed by default instead of auto-expanding.
• "single": a single fixed-size line pointing at the immediate parent class only, regardless of how many methods are inherited or how deep the chain is -- the actual bloat fix, for packages where even the grouped form is too large.
• "original": byte-for-byte the previous rendering, for anyone who prefers it or hits a regression.

All three were verified against roxygen2's own test suite (8 new/updated tests, plus updated snapshots for the two existing integration tests that exercise multi-level R6 inheritance) and against a real-world package with R6 inheritance chains up to 5 levels deep -- "original" mode produced a total generated man/ size of ~5.0M, "grouped" reduced that to ~4.1M (same information, just organized by ancestor and collapsed by default instead of flattened and auto-expanded), and "single" reduced it further to ~2.2M by dropping per-method enumeration entirely.

This changes the default rendering (flat list -> grouped-by-ancestor, collapsed instead of auto-expanded) but not the underlying information shown; original is available for anyone who wants the old default back exactly.