fix(scripts): add_deprecations.sh generates additive aliases for @[to_additive] decls

[syedjafri06193]

Fixes #38550

When a renamed declaration is preceded by @[to_additive], the script
previously only emitted a deprecation alias for the multiplicative name.
This PR also emits one for the additive counterpart.

Changes:

• Switch git diff to --unified=1 so the unchanged @[to_additive]
  attribute appears as a context line in the diff output
• Track that context line in awk; when detected, apply standard
  mul→add word substitutions to derive additive old/new names and
  emit a second @[deprecated] alias

Example — before:
@[deprecated (since := "2026-06-04")] alias foo_mul := bar_mul

Example — after:
@[deprecated (since := "2026-06-04")] alias foo_mul := bar_mul
@[deprecated (since := "2026-06-04")] alias foo_add := bar_add

---

AI disclosure: this fix was developed with Claude (claude.ai).
I understand all the changes and can explain every design decision.

[github-actions]

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[github-actions]

PR summary d173b20fc5

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

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 d173b20).

• +0 new declarations
• −0 removed declarations

No declaration differences.

---

No changes to strong technical debt.

No changes to weak technical debt.

Current commit d173b20fc5
Reference commit 3e6cc1a38a

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 for your contribution. I agree that there is a strong need for functionality like this. That said, the guessing of the new declaration's name is quite ad hoc --- I worry about this being brittle, and wonder if there's a more robust way to do this.

For instance: @adomani can we use the declaration diff data to generate these deprecations form within Lean, replacing awk (which is... not as widely known) with Lean code?

[grunweg]

How have to determined these patterns? Taking the most common ones from to_additive? Is this list exhaustive? Are there edge cases where this produces wrong results?