feat(RepresentationTheory): add Subrepresentation.toRepresentation_apply and Subrepresentation.quotient

[JX-Mo]

Add a lemma for Subrepresentation recording the inherited group action. Help avoid unfold Subrepresentation.toRepresentation.

Add the quotient representation associated to a subrepresentaiton. It wraps up Representation.quotient and Representation.Subrepresentation. Help avoid writing down explicitly the stability condition required in Representation.quotient.

[github-actions]

Welcome new contributor!

Thank you for contributing to Mathlib! If you haven't done so already, please review our contribution guidelines, as well as the style guide and naming conventions. In particular, we kindly remind contributors that we have guidelines regarding the use of AI when making pull requests.

We use a review queue to manage reviews. If your PR does not appear there, it is probably because it is not successfully building (i.e., it doesn't have a green checkmark), has the awaiting-author tag, or another reason described in the Lifecycle of a PR. The review dashboard has a dedicated webpage which shows whether your PR is on the review queue, and (if not), why.

If you haven't already done so, please come to https://leanprover.zulipchat.com/, introduce yourself, and mention your new PR.

Thank you again for joining our community.

[github-actions]

PR summary a96b6d046c

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff (regex)

+ quotient
+ quotient_apply
+ toRepresentation_apply

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 a96b6d0).

• +3 new declarations
• −0 removed declarations

+Subrepresentation.quotient
+Subrepresentation.quotient_apply
+Subrepresentation.toRepresentation_apply

---

No changes to strong technical debt.

No changes to weak technical debt.

Current commit a96b6d046c
Reference commit 571b8a8e54

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!

[JX-Mo]

"easy"
"awaiting-review"

[Whysoserioushah]

easy means less than 20s of review time, this PR clearly takes a bit more :-)