feat(Analysis/Fourier/AddCircle): add evaluation lemmas for fourier characters

[j-white]

Add the argument-side counterparts of the index lemmas (fourier_add / fourier_neg):

• fourier_eval_add
• fourier_eval_neg
• fourier_eval_sub

Additionally, add the index-subtraction lemma fourier_sub (with simp normal form fourier_sub', mirroring fourier_add').

[github-actions]

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[github-actions]

PR summary 136285cf26

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff (regex)

+ fourier_eval_add
+ fourier_eval_neg
+ fourier_eval_sub
+ fourier_sub
+ fourier_sub'

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 -- stale, waiting for the new build)

✅ Lean-aware diff — post-build, computed from the Lean environment (commit 327c0f3).

• +5 new declarations
• −0 removed declarations

+fourier_eval_add
+fourier_eval_neg
+fourier_eval_sub
+fourier_sub
+fourier_sub'

---

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

Current commit 136285cf26
Reference commit 3357482abc

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).