chore(toolkit-lib): use pre-built cdk.out fixtures in deploy and diff tests for speed

[mrgrain]

Speeds up deploy.test.ts and diff.test.ts by eliminating ~34 redundant CDK app syntheses per test run.

Each builderFixture call was running a full synthesis through fromAssemblyBuilder, loading aws-cdk-lib modules, creating temp directories, acquiring file locks, and resolving the default AWS environment. None of these tests actually test synthesis — they only care about the resulting CloudFormation templates and manifest metadata.

All builderFixture calls are replaced with cdkOutFixture, which reads pre-built cdk.out directories from disk via fromAssemblyDirectory. A new gen-fixture projen task and script (scripts/gen-fixture-cdk-out.ts) are added to generate and regenerate these fixtures using Toolkit.fromAssemblyBuilder, ensuring the output is identical to what the tests were previously synthesizing at runtime.

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[github-actions]

Total lines changed 4012 is greater than 1000. Please consider breaking this PR down.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.20%. Comparing base (ae062a9) to head (33426cf).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1301   +/-   ##
=======================================
  Coverage   88.19%   88.20%           
=======================================
  Files          73       73           
  Lines       10386    10386           
  Branches     1410     1409    -1     
=======================================
+ Hits         9160     9161    +1     
+ Misses       1199     1198    -1     
  Partials       27       27           

Flag	Coverage Δ
suite.unit	88.20% <ø> (+<0.01%)	⬆️

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