feat(formatter): place multiline method return types

[floitsch]

Part 5 of the formatter rewrite stack. Based on #3139.

This isolates the formatter's one token-order exception in format_return_type.cc.

Generic method lowering still constructs and first selects the ordinary source-order header. Only when the selected parameter span is multiline does this pass construct the explicit return-first order:

foo -> ReturnType
    first
    second
:

Flat parameters retain the ordinary form:

foo first second -> ReturnType:

The exception consumes private semantic method pieces through the single select_method_header entry point. No general Layout, SelectedPlan, or repair API gains a relocation operation, and no other formatter component can assemble an alternative method-header order.

The method tests reparse both ordinary and reordered headers, compare their semantic header trees, and format them a second time to verify idempotence. Compiler analysis also passes for both formatter fixtures.

Stack: 5/5.