feat(parser): add --flag=value syntax and unknown flag handling

[napalu]

• Support standard --flag=value and -f=value formats for all flag types
• Preserve internal '=' signs in values (URLs, env vars, JSON, etc.)
• Add config options to allow or treat unknown flags as positionals
• Fix i18n code generation bugs and key normalization
• Update and extend tests (34 new cases, fuzz coverage)

Co-developed with Claude (AI-assisted)

[codecov]

Codecov Report

❌ Patch coverage is 66.82028% with 72 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.93%. Comparing base (1970cca) to head (5f77ebb).
⚠️ Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
v2/goopt_internal.go	49.19%	48 Missing and 15 partials ⚠️
v2/goopt.go	82.35%	4 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #128      +/-   ##
==========================================
+ Coverage   79.29%   79.93%   +0.63%     
==========================================
  Files          88       88              
  Lines       16562    13190    -3372     
==========================================
- Hits        13133    10543    -2590     
+ Misses       2725     1924     -801     
- Partials      704      723      +19     

Flag	Coverage Δ
v1	74.74% <100.00%> (+1.82%)	⬆️
v2	82.82% <66.66%> (+0.06%)	⬆️

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

Files with missing lines	Coverage Δ
goopt.go	74.03% <100.00%> (+1.80%)	⬆️
v2/goopt_internal_positionals.go	97.08% <100.00%> (+0.44%)	⬆️
v2/i18n/bundle.go	72.69% <100.00%> (+0.90%)	⬆️
v2/parser_config_funcs.go	94.92% <100.00%> (+3.16%)	⬆️
v2/renderer.go	99.09% <100.00%> (+2.08%)	⬆️
v2/goopt.go	87.65% <82.35%> (+0.54%)	⬆️
v2/goopt_internal.go	75.01% <49.19%> (-0.79%)	⬇️

... and 79 files with indirect coverage changes

Files with missing lines	Coverage Δ
goopt.go	74.03% <100.00%> (+1.80%)	⬆️
v2/goopt_internal_positionals.go	97.08% <100.00%> (+0.44%)	⬆️
v2/i18n/bundle.go	72.69% <100.00%> (+0.90%)	⬆️
v2/parser_config_funcs.go	94.92% <100.00%> (+3.16%)	⬆️
v2/renderer.go	99.09% <100.00%> (+2.08%)	⬆️
v2/goopt.go	87.65% <82.35%> (+0.54%)	⬆️
v2/goopt_internal.go	75.01% <49.19%> (-0.79%)	⬇️

... and 79 files with indirect coverage changes

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