feat(Data/ENNReal/Inv): add ENNReal.div_mul_div_cancel

[jdhart81]

Proves a / b * (b / c) = a / c for b ≠ 0, b ≠ ∞ in ENNReal. Proof via mul_div_assoc and existing div_mul_cancel.

AI Disclosure: This PR was developed with assistance from an LLM (Claude, Anthropic) for proof exploration and text drafting. The proof was compiled and verified locally by the contributor, who understands the mathematical content. This lemma is extracted from a larger formalization of information-theoretic bounds (Zenodo DOI: 10.5281/zenodo.19317983).

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[github-actions]

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[github-actions]

PR summary b73ba7707a

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

+ div_mul_div_cancel

You can run this locally as follows

## summary with just the declaration names:
./scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh contains some details about this script.

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No changes to technical debt.

You can run this locally as

./scripts/reporting/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).