Dedup: extract shared _all_elim_str helper for AllElim/AllElimTypes (refs #813)

[jsiek]

Summary

Small duplication-reduction slice for the #813 umbrella.

AllElim.__str__ and AllElimTypes.__str__ in abstract_syntax/proofs.py
were byte-for-byte identical except for the two bracket glyphs — [/] for
term arguments vs </> for type arguments. Both already delegated their
base-proof rendering to the module-level _postfix_base_str.

This PR folds the shared control flow into a new module-level helper next to
_postfix_base_str:

def _all_elim_str(univ, arg, pos, open_b, close_b) -> str: ...

so each __str__ becomes a one-line call:

• AllElimTypes.__str__ → _all_elim_str(self.univ, self.arg, self.pos, "<", ">")
• AllElim.__str__ → _all_elim_str(self.univ, self.arg, self.pos, "[", "]")

No behavior change: the emitted string is identical for both cases.

Tests

Ran locally (green):

• python3 test-deduce.py --equiv — RD/LALR AST equivalence and the
  pretty-print round-trip corpus, which exercises AllElim/AllElimTypes
  printing directly.
• python3 test-deduce.py --passable — should-validate under both parsers.

ruff and mypy are not installed in this container; CI's static-checks leg
covers them. The change is a pure refactor with no signature changes to public
APIs.

Refs #813.

Resume this session on ginger: ~/deduce-runner/resume.sh 208dc587-9660-431b-b3f2-a9d904333886 routine/20260728-010202

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