feat(Algebra/MvPolynomial): add coefficient lemma for uniqueAlgEquiv

[eliasjudin]

Summary

• depends on: [Merged by Bors] - refactor(Algebra/MvPolynomial): generalize pUnitAlgEquiv to Unique #37801

Adds MvPolynomial.coeff_uniqueAlgEquiv, identifying the coefficient of X ^ n in
MvPolynomial.uniqueAlgEquiv R σ P with the coefficient of the unique multivariate monomial
Finsupp.single default n in P.

Motivation

This splits the coefficient-level API for MvPolynomial.uniqueAlgEquiv out of #37801, following
Eric Wieser's review request.

[github-actions]

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[mathlib-dependent-issues]

This PR/issue depends on:

• [Merged by Bors] - refactor(Algebra/MvPolynomial): generalize pUnitAlgEquiv to Unique #37801
  By Dependent Issues (🤖). Happy coding!

[github-actions]

PR summary 400c83017d

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff

+ coeff_uniqueAlgEquiv

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.

---

No changes to technical debt.

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).

[eric-wieser]

Please skip the summary heading in this and future PR descriptions. Implicitly the introductory text is the summary.

[eric-wieser]

Could you add the symm version too?