feat(SimpleGraph): cycleGraph_EulerianCircuit is a cycle

[vlad902]

(A follow-up PR will show that cycleGraph_EulerianCircuit is also hamiltonian.)

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• depends on: [Merged by Bors] - feat(SimpleGraph): alternative characterizations of cycles/hamiltonian paths #34766
• depends on: [Merged by Bors] - feat(SimpleGraph): simp lemmas for first and last support elements #34796

[github-actions]

PR summary 81b174f9de

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

+ cycleGraph_EulerianCircuit_cons_getVert
+ cycleGraph_EulerianCircuit_getVert
+ cycleGraph_EulerianCircuit_isCycle
+ cycleGraph_EulerianCircuit_tail_isPath

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).

[mathlib-dependent-issues]

This PR/issue depends on:

• [Merged by Bors] - feat(SimpleGraph): alternative characterizations of cycles/hamiltonian paths #34766
• [Merged by Bors] - feat(SimpleGraph): simp lemmas for first and last support elements #34796
  By Dependent Issues (🤖). Happy coding!

[Ruben-VandeVelde]

This is misnamed - there should be no underscore in the name of a def. I propose renaming it and everything that refers to it (with deprecation) before adding more misnamed theorems.

[vlad902]

#35360