Compute Context.module_names lazily to avoid rescan on every evolve()

[sir-sigurd]

Issue #, if available: #4585

Description of changes:

Context.__post_init__ rebuilt module_names on every Context construction — including all evolve() clones created during the schema walk — making validation O(nodes × resources) on every template (#4510, first released in 1.51.1). See the linked issue for profiles: on a ~250 KB template with ~270 resources this is ~84k constructions triggering ~22.4M str.endswith calls, ~46% of total runtime.

This change replaces the eager field with functools.cached_property, so the tuple is computed at most once per instance and only when actually read. Both readers (dynamicValidation in _keywords_cfn.py and GetAtt) consult it only at specific nodes, so most instances never compute it at all. cached_property works on the frozen dataclass because it writes to the instance __dict__ directly, bypassing the frozen __setattr__.

Behavior note: module_names is no longer a dataclass field, so it is not part of the generated __eq__/__repr__. It is fully derived from resources (which is a field), and nothing in the codebase introspects it as a field.

Benchmarks (Apple M-series, warm): synthetic 400-resource template 4.8s → 3.7s (pre-regression 1.51.0 baseline: 3.4s); a fleet of 66 large (~250 KB) generated templates 3m55s → 2m42s. Unit test suite and pre-commit hooks pass.

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Prepared with AI assistance (Claude Code); human-reviewed before submission.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.42%. Comparing base (9c2445f) to head (7ed69ee).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4586      +/-   ##
==========================================
- Coverage   94.43%   94.42%   -0.01%     
==========================================
  Files         429      429              
  Lines       15099    15099              
  Branches     2916     2916              
==========================================
- Hits        14258    14257       -1     
  Misses        461      461              
- Partials      380      381       +1     

Flag	Coverage Δ
unittests	94.42% <100.00%> (-0.01%)	⬇️

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