feat: adversarial synthetic generator for ImpactGuard reliability testing

[Copilot]

Adds a synthetic generator that produces signature pairs where a breaking API change is camouflaged as a non-breaking one, stress-testing whether ImpactGuard's detection is reliable against deceptive diffs.

New module: adversarial_generator.py

10 camouflage strategies, each producing an AdversarialPair with old/new signature snapshots, expected breaking patterns, and human-readable deception notes:

Strategy	Hidden breaking change	Camouflage
required_param_hidden_by_type_annotation	Required positional added	str → str | None widening on sibling param
positional_reorder_hidden_by_optional_add	Param order swapped	Optional third param added
type_narrowing_disguised_as_cleanup	str | None → str narrowing	Looks like type-safety tightening
kwonly_removal_with_optional_addition	Required kwarg removed	Optional kwarg added
vararg_removal_with_kwarg_addition	*args removed	**kwargs added
return_type_narrowing_disguised_as_guarantee	str | None → str return	"Now always returns a value"
decorator_removal_hidden_by_optional_kwarg	@cache removed	Optional kwarg suggests cache control
required_kwonly_added_with_misleading_name	Required kwarg added	Named optional_context with no default
function_removal_hidden_by_rename_addition	Old function removed	_v2 variant added alongside
kwargs_removal_hidden_by_explicit_params	**kwargs removed	Explicit optional params added

from impactguard import generate_adversarial, generate_all_adversarial, list_adversarial_strategies

pair = generate_adversarial("required_kwonly_added_with_misleading_name")
# pair.old_signatures / pair.new_signatures → pass to compare()
# pair.expected_breaking_patterns → what ImpactGuard must catch
# pair.camouflage_notes → explains the deception

ok, failures = pair.verify(compare(old_path, new_path))

Tests (test_adversarial_generator.py)

54 tests verifying ImpactGuard catches every hidden breaking change — including parametrized bulk detection across all 10 strategies, camouflage noise presence checks, and AdversarialPair.verify() unit tests.

Exports

AdversarialPair, generate_adversarial, generate_all_adversarial, list_adversarial_strategies added to __init__.py / __all__.

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[codacy-production]

Not up to standards ⛔

🔴 Issues 24 high · 21 minor

Alerts:
⚠ 45 issues (≤ 0 issues of at least minor severity)

Results:
45 new issues

Category	Results
UnusedCode	1 minor
Documentation	20 minor
Security	24 high

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🟢 Metrics 89 complexity · 2 duplication

Metric	Results
Complexity	89
Duplication	2

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