feat(CategoryTheory/Limits): chosen pullbacks

[joelriou]

Given two morphisms f₁ : X₁ ⟶ S and f₂ : X₂ ⟶ S, we introduce a structure ChosenPullback f₁ f₂ which contains the data of a pullback of f₁ and f₂. A variant for triple wide pullbacks is also introduced.

This will be used in the study of the effectiveness of the descent #24434.

Co-authored-by: Christian Merten

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[github-actions]

PR summary 99884c2730

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference
Mathlib.CategoryTheory.Limits.Shapes.Pullback.ChosenPullback (new file)	601

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Declarations diff

+ ChosenPullback
+ ChosenPullback₃
+ Diagonal
+ LiftStruct
+ exists_lift
+ hom_ext
+ hp₂
+ instance : Nonempty h.Diagonal := by apply LiftStruct.nonempty <;> cat_disch
+ instance : Subsingleton (h.LiftStruct g₁ g₂ b)
+ isPullback
+ isPullback₁
+ isPullback₂
+ isPullback₃
+ mk'
+ nonempty
+ pullback
+ p₁₂
+ p₁₂_eq_lift
+ p₁₂_p
+ p₁₂_p₁
+ p₁₂_p₂
+ p₁₃
+ p₁₃_eq_lift
+ p₁₃_p
+ p₁₃_p₁
+ p₁₃_p₃
+ p₂
+ p₂₃
+ p₂₃_eq_lift
+ p₂₃_p
+ p₂₃_p₂
+ p₂₃_p₃
+ w
+ w₁
+ w₂
+ w₃

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).