trait solver: Resolve region vars before eager placeholder handling

[Dnreikronos]

Region vars can already have known values in the unification table while the stored constraint still contains the vars. max_universe looks through the table, but PlaceholderReplacer sees the stored terms. In Boxy's nested binder case, that leaves an alias constraint ambiguous even after equating its regions.

I used deeply_resolve_via_unification_table at the start of eager handling for each universe. That leaves only the current universe's placeholder to bind and lets the result match the actual assumption on the outer binder. Unknown vars stay unknown.

The test DSL needed a small equality operation, 'a2 = 'a, to reach this state. The paired outlives constraints in the original example go straight into solver storage and don't populate the table on current main. I kept that example as an ambiguity control and added the version with real table equalities, plus a chained-var case. Both adjusted cases fail when the resolution call is removed and pass with it.

I think keeping this separate makes #157984 easier to review. The broader equality and subtyping work stays there.

[rustbot]

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

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[Dnreikronos]

r? @BoxyUwU

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