feat(SimpleGraph): add max-flow/min-cut weak duality

[floor-licker]

This PR introduces a basic s–t flow setup for undirected SimpleGraphs and proves the standard weak-duality
inequality: for any feasible flow f and any s–t cut S, value f ≤ cutCapacity S.

This is a small, self-contained lemma I extracted while working on larger graph-theoretic formalizations, in particular, results that will ultimately rely on a full max-flow/min-cut theorem. The full MFMC equality/existence statement is not included here. This is the weak-duality direction (∀ f S, value f ≤ cutCapacity S).

[github-actions]

PR summary 3de2350c1f

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference
Mathlib.Combinatorics.SimpleGraph.Connectivity.MaxFlowMinCut (new file)	587

---

Declarations diff

+ Flow
+ cutCapacity
+ cutValue
+ cutValue_le_cutCapacity
+ netOut
+ netOut_def
+ value
+ value_def
+ value_eq_cutValue
+ value_le_cutCapacity

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 script/declarations_diff.sh contains some details about this script.

---

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).

[github-actions]

✅ PR Title Formatted Correctly

The title of this PR has been updated to match our commit style conventions.
Thank you!