Add type groups proposal

[MadsTorgersen]

Summary

• introduce type groups as shared, method-group-like lookup infrastructure for same-name accessible types across generic arities
• define type-group-name syntax, cross-arity lookup, singleton ordinary types, accessibility, aliases, stopping, nested hiding, and same-arity ambiguity
• keep inference, constructors, applicability, overload resolution, and type-pattern consumption out of scope

Shared proposal series

Layer	csharplang proposal	csharpstandard specification
Generalized inference	#10311	#1
Type groups	#10310	#2
Constructor inference	#10312	#3

The csharplang proposal PRs are independent siblings based directly on main. The csharpstandard PRs are mechanically stacked in the order #1 → #2 → #3. Type groups are conceptually independent lookup infrastructure; constructor inference is their first consumer.

Champion issue

There is no known dedicated champion issue for this shared infrastructure. Issue ownership may need either a dedicated champion issue or a provisional association with constructor inference #9627.

Validation

• git diff --check
• reviewed against Design-Process.md and proposals/proposal-template.md
• confirmed the proposal contains only proposals/type-groups.md and excludes obsolete draft terminology and design