feat(Data/Set/Basic): add some missing lemmas

[joneugster]

This PR continues the work from #22837.

Original PR: #22837

[joneugster]

Comments from Original PR #22837

This section contains 1 comment(s) from the original PR, excluding bot comments.

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@mathlib-merge-conflicts (2026-02-06 15:21 UTC):
This pull request has conflicts, please merge master and resolve them.

[github-actions]

PR summary 88bb6d226a

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

+ subset_iff
++ subset_iff_notMem

You can run this locally as follows

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

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

The doc-module for scripts/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/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).

[vihdzp]

This is a duplicate of subset_def