[ffigen] Support running as a standalone AOT executable

[sigurdm]

Declare ffigen under executables in pubspec.yaml and resolve the dart executable via package:cli_util (cli_util: ^0.6.0) instead of walking parent directories of Platform.resolvedExecutable.

When ffigen is compiled to a standalone AOT executable (for example via dart install ffigen or dart run ffigen@), Platform.resolvedExecutable is the ffigen binary itself rather than <sdk>/bin/dart, which previously caused dartExecutable discovery for dart format to fail. Also updates findPackageRoot in overrideable_utils.dart to handle non-.dart/.dill Platform.script URIs and filesystem root traversal safely.

[codecov]

Codecov Report

❌ Patch coverage is 82.35294% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.02%. Comparing base (3cbe319) to head (987224c).

Files with missing lines	Patch %	Lines
pkgs/ffigen/lib/src/code_generator/utils.dart	50.00%	2 Missing ⚠️
...en/lib/src/config_provider/overrideable_utils.dart	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3746      +/-   ##
==========================================
+ Coverage   87.94%   88.02%   +0.07%     
==========================================
  Files         310      310              
  Lines       19848    19842       -6     
==========================================
+ Hits        17456    17465       +9     
+ Misses       2392     2377      -15     

Flag	Coverage Δ
ffigen	91.55% <82.35%> (+0.19%)	⬆️
native_pkgs_macos	86.47% <ø> (ø)
native_pkgs_ubuntu	72.14% <ø> (ø)
native_pkgs_windows	75.47% <ø> (ø)
objective_c	62.91% <0.00%> (+0.04%)	⬆️
swiftgen	80.41% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
code_assets	94.96% <ø> (ø)
data_assets	91.56% <ø> (ø)
ffi	100.00% <ø> (ø)
ffigen	91.65% <82.35%> (+0.19%)	⬆️
hooks	86.40% <ø> (ø)
hooks_runner	90.52% <ø> (ø)
jni	66.34% <ø> (ø)
jni_flutter	∅ <ø> (∅)
jni_util	25.00% <ø> (ø)
jnigen	82.85% <ø> (ø)
json_syntax_generator	∅ <ø> (∅)
native_test_helpers	85.71% <ø> (ø)
native_toolchain_c	95.97% <ø> (ø)
objective_c	79.53% <ø> (ø)
pub_formats	∅ <ø> (∅)
record_use	81.48% <ø> (ø)
snippet_tool	66.23% <ø> (ø)
swift2objc	92.95% <ø> (ø)
swiftgen	80.41% <ø> (ø)
test_case_selector	97.28% <ø> (ø)

Files with missing lines	Coverage Δ
...en/lib/src/config_provider/overrideable_utils.dart	86.48% <92.30%> (+28.15%)	⬆️
pkgs/ffigen/lib/src/code_generator/utils.dart	94.87% <50.00%> (+10.08%)	⬆️

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