chore(Algebra/QuadraticAlgebra): deprecate the old Discr module

[xroblot]

Re-adds Mathlib/Algebra/QuadraticAlgebra/Discr.lean as a deprecated_module shim redirecting to the renamed Discriminant module (see #42975).

Prepared with Claude Code 🤖

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[github-actions]

PR summary 525af60fd7

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference
Mathlib.Algebra.QuadraticAlgebra.Discr	1
Mathlib.Algebra.QuadraticAlgebra.Discriminant (new file)	1372

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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 525af60).

• +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 525af60fd7
Reference commit ffbfefaec6

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).

[grunweg]

Thanks!
bors stack #42976

[mathlib-bors]

👎 Cannot stack this pull request on itself. Name the pull request it builds on, e.g. bors stack #123.

[grunweg]

bors stack 42975

[mathlib-bors]

#42976 is now stacked on #42975: both are in a linked bundle and merge in the same batch, or not at all. The batch applies #42975's changes first (#42976's own changes). Each pull request still needs its own bors r+; bors unlink removes the link.

View this bundle in bors (sign in with GitHub to view).

[grunweg]

bors stack #42975

[mathlib-bors]

#42976 is now stacked on #42975: both are in a linked bundle and merge in the same batch, or not at all. The batch applies #42975's changes first (#42976's own changes). Each pull request still needs its own bors r+; bors unlink removes the link.

View this bundle in bors (sign in with GitHub to view).

[mathlib-bors]

The rest of the bundle is approved; it enters the queue once this pull request gets bors r+.

View this bundle in bors (sign in with GitHub to view).

[grunweg]

bors merge

[mathlib-bors]

👎 Rejected by label

[xroblot]

@grunweg Maybe the fact that this one is marked as blocked by other PR is causing the troubles. I'll remove it