chore(Algebra/QuadraticAlgebra): define natCast and intCast without C

[xroblot]

The natCast and intCast fields go through QuadraticAlgebra.C, which is opaque at implicit transparency, so Ring.toIntAlgebra and instAlgebra are not defeq for rw. Writing them as ⟨n, 0⟩ fixes it, as #43242 did for algebraMap.

Prepared with Claude Code 🤖

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[github-actions]

PR summary 40e78fbe18

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff (regex)

No declarations were harmed in the making of this PR! 🐙

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 40e78fb).

• +0 new declarations
• −0 removed declarations

No declaration differences.

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No changes to strong technical debt.
No changes to weak technical debt.

Current commit 40e78fbe18
Reference commit 9b46e33ce0

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.py 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).

[xroblot]

!radar

[jcommelin]

Thanks 🎉

bors merge

[mathlib-bors]

👎 Rejected by label

[jcommelin]

Whhops, too fast. But I do like the look of this.

[xroblot]

@jcommelin I still want to do some testing before merging this PR

[leanprover-radar]

Benchmark results for 40e78fb against 9b46e33 are in. No significant results found. @xroblot

• ✅ build//instructions: -1.4G (-0.00%)

No significant changes detected.

[xroblot]

@jcommelin Timings are fine so this PR is ready to be merged if you want to