chore: move simple_obj into Functor namespace, golf proofs, tidy docstring

[kim-em]

This PR moves CategoryTheory.simple_obj and CategoryTheory.simple_obj_iff into the Functor namespace (matching their sibling Functor.simple_of_simple_obj and enabling dot notation), golf Functor.simple_obj and the forward direction of isSimpleModule_iff_eq_zero_or_injective via the existing LinearMap.injective_or_eq_zero, remove unused variable binders in RingTheory/SimpleRing/DivisionRing.lean, and fix inverted heading levels and an "an unique" typo in that file's module docstring. The one call site of the renamed lemmas is updated; no deprecated aliases are added since the declarations merged a week ago.

Follow-up to #41233 (feat(SimpleRing/DivisionRing): simple module is preserved by ModuleCat equivs).

🤖 Prepared with Claude Code

[github-actions]

PR summary 65a3e33478

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff (regex)

+ Functor.simple_obj
+ Functor.simple_obj_iff
- simple_obj
- simple_obj_iff

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 65a3e33).

• +2 new declarations
• −2 removed declarations

+CategoryTheory.Functor.simple_obj
+CategoryTheory.Functor.simple_obj_iff
-CategoryTheory.simple_obj
-CategoryTheory.simple_obj_iff

---

No changes to strong technical debt.

No changes to weak technical debt.

Current commit 65a3e33478
Reference commit a33a5ccd63

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).