feat(cli): add --concurrency flag to cdk destroy

[mikevoets]

Fixes #752

Description

Add --concurrency flag to cdk destroy, mirroring the existing cdk deploy --concurrency behavior. When set to a value greater than 1, multiple stacks are destroyed in parallel while respecting dependency order (reversed from deploy).

Uses WorkGraph with reversed dependency edges to orchestrate parallel destruction. A new WorkGraph.processStacks() method provides a stack-only execution path, and the graph construction logic lives in its own build-destroy-work-graph module.

Checklist

• This change contains a major version upgrade for a dependency and I confirm all breaking changes are addressed
  • Release notes for the new version:

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[mrgrain]

Nice one!

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.05%. Comparing base (cf3cd7d) to head (25be8f0).

Files with missing lines	Patch %	Lines
packages/aws-cdk/lib/cli/cdk-toolkit.ts	95.45%	1 Missing ⚠️
packages/aws-cdk/lib/cli/cli.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1221      +/-   ##
==========================================
+ Coverage   87.79%   88.05%   +0.25%     
==========================================
  Files          73       73              
  Lines       10168    10187      +19     
  Branches     1343     1351       +8     
==========================================
+ Hits         8927     8970      +43     
+ Misses       1217     1193      -24     
  Partials       24       24              

Flag	Coverage Δ
suite.unit	88.05% <91.66%> (+0.25%)	⬆️

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