feat(Analysis/CStarAlgebra/Matrix): add CStarAlgebra instance for matrices

[judsonpereirademoura-netizen]

Adds the bundled CStarAlgebra instance for Matrix n n ℂ in the L2 operator norm (Mathlib already has CStarRing scoped in Matrix.Norms.L2Operator, but not the bundled algebra). Composed from the existing scoped instances. Follow-up to #40900.

[github-actions]

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[github-actions]

PR summary 59bc3226f8

Import changes for modified files

Dependency changes

File	Base Count	Head Count	Change
Mathlib.Analysis.CStarAlgebra.Matrix	2210	2214	+4 (+0.18%)

Import changes for all files

Files	Import difference
6 files Mathlib.Analysis.CStarAlgebra.Matrix Mathlib.Analysis.Convex.Birkhoff Mathlib.Analysis.Convex.DoublyStochasticMatrix Mathlib.LinearAlgebra.Matrix.Permutation Mathlib.LinearAlgebra.Matrix.Stochastic Mathlib.LinearAlgebra.Matrix.Swap	4

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Declarations diff (regex)

+ instCStarAlgebra

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 59bc322).

• +1 new declarations
• −0 removed declarations

+Matrix.instCStarAlgebra

---

No changes to strong technical debt.

No changes to weak technical debt.

Current commit 59bc3226f8
Reference commit 80ffd59621

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).

[github-actions]

✅ PR Title Formatted Correctly

The title of this PR has been updated to match our commit style conventions.
Thank you!

[judsonpereirademoura-netizen]

Thanks for the review! Applied the where suggestion.