chore(Topology/CantorBendixson): cleanup + more basic API

[vihdzp]

We perform various small golfs, make some variables explicit, and add a few trivial lemmas.

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I would've suggested all of this in #37375, but somehow that PR went under my radar.

[github-actions]

PR summary 5ab0ff5144

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 (regex)

+ gfpApprox_def
+ iteratedDerivSet_of_ne_zero
+ iteratedDerivedSet_add_one
+ iteratedDerivedSet_def
+ iteratedDerivedSet_monotone
+ lfpApprox_def

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 5ab0ff5).

• +6 new declarations
• −0 removed declarations

+CantorBendixson.iteratedDerivSet_of_ne_zero
+CantorBendixson.iteratedDerivedSet_add_one
+CantorBendixson.iteratedDerivedSet_def
+CantorBendixson.iteratedDerivedSet_monotone
+OrdinalApprox.gfpApprox_def
+OrdinalApprox.lfpApprox_def

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No changes to strong technical debt.
No changes to weak technical debt.

Current commit 5ab0ff5144
Reference commit e3ea2ac394

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.py 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).