refactor(QED): remove erw from sum lemmas

[catlover-bot]

Summary

This PR removes the two remaining uses of erw from
Physlib/QFT/QED/AnomalyCancellation/Basic.lean.

The affected lemmas are:

• PureU1.sum_of_charges
• PureU1.sum_of_anomaly_free_linear

In both proofs, the previous erw relied on definitional equality while applying the induction hypothesis. The new proofs make that step explicit with change, instantiate the induction hypothesis directly, and use congrArg to add the final summand to both sides.

No lemma statements or mathematical content are changed.

Validation

• lake build +Physlib.QFT.QED.AnomalyCancellation.Basic
• full LEAN_NUM_THREADS=1 lake build (9269 jobs)
• direct Lean check with linter.style.multiGoal and linter.style.longLine
• git diff --check
• ./scripts/lint-style.sh
• confirmed no erw remains in the modified file
• lake exe lint_all: build/import/sorry/Lean linters pass; repository-wide pre-existing style and transitive-import issues remain in unrelated files

Contributes to #385.

[github-actions]

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[jstoobysmith]

Looks good - many thanks. Approved

[catlover-bot]

Thanks for the review!