[Tracking] ReadyToRun Improvements for .NET 12

In .NET 12, we are looking at making improvements in Crossgen2 to further reduce startup/tier up time, memory usage, and the number of methods we need to JIT for interesting scenarios.

Additionally, crossgen2 has a number of different knobs and options with various guarantees around functionality as well as many implicit assumptions and incompatibilities for said features. We plan to simplify our story here to provide a well-understood experience with known trade-offs at each tier.

## dotnet/runtime work

- [ ] Document the various "more than one assembly" version bubble options and their limitations and support stories.
- [ ] Add support for cross-module inlining and cross-module generics with composite mode.
- [ ] ELF Container support for R2R
- [ ] WebCIL R2R improvements

## Partner work (proposed)

- [ ] Enable "larger than one assembly version bubble" support for various Roslyn compiler components in the .NET SDK
- [ ] Ensure the roslyn LSP has fast enough startup that AI queries to the LSP are "worthwhile" on a token vs speed tradeoff

Start by reviewing the Crossgen2 and ReadyToRun work described in the tracking list, then determine which specific dotnet/runtime item has an agreed scope and entry point. Done would require a separately defined implementation task with explicit limitations, tests, and acceptance criteria; this issue alone does not provide them.

Written by the indexing model from the issue text.