feat(Data/Finset/Powerset): disjointness lemmas for powersetCard

[mo271]

Add disjointness lemmas for Finset.powersetCard:

• Disjoint.powersetCard_powersetCard: powersetCard of disjoint sets are disjoint for positive cardinality
• Finset.disjoint_powersetCard_of_ne: powersetCard of distinct cardinalities are disjoint
• Simplify some proofs using disjoint_powersetCard_of_ne

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[github-actions]

PR summary a8573a7e4c

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff (regex)

+ Disjoint.powersetCard_powersetCard
+ disjoint_powersetCard_of_ne

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

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

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

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

Declarations diff (Lean)

✅ Lean-aware diff — post-build, computed from the Lean environment (commit a8573a7).

• +2 new declarations
• −0 removed declarations

+Finset.Disjoint.powersetCard_powersetCard
+Finset.disjoint_powersetCard_of_ne

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No changes to strong technical debt.

No changes to weak technical debt.

Current commit a8573a7e4c
Reference commit e506e4611d

This script lives in the mathlib-ci repository. To run it locally, from your mathlib4 directory:

git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci
../mathlib-ci/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).