refactor(all): mark enum-like DUs [<Struct>] to satisfy StructDiscriminatedUnionAnalyzer

[dbrattli]

Summary

Resolves the editable IONIDE-012 (StructDiscriminatedUnionAnalyzer) code scanning alerts by marking the flagged discriminated unions [<Struct>] — the same pattern already used for FieldNamingKind, ClassStyle, and Atom.

62 alerts across 20 files are addressed:

• The majority are enum-like / single-case / already-named-field DUs needing only the [<Struct>] attribute (bulk in Rust.AST.Types.fs ×26, Python.AST.fs ×6, Global/Babel.fs ×6, Dart/Dart.fs ×4, …).
• 8 multicase DUs with unnamed fields also required unique field names to satisfy the struct-union rule (FS3585). Positional pattern matching is preserved, so call sites are unaffected:
  • Php.fs: PhpConst, PhpArrayIndex, Capture, PhpCase
  • Fable2Php.fs: ReturnStrategy
  • FSharp2Fable.Util.fs: CompiledValue
  • Globbing.fs (+ its .fsi signature): SearchOption
  • Util.fs: AttParam

Left untouched

The 8 IONIDE-012 alerts in src/Fable.AST/ (Common.fs, Fable.fs, Plugins.fs) are intentionally not changed. Adding [<Struct>] there would flip a public type from a reference type to a value type — a breaking change disallowed for that project.

Verification

• Fable.Cli (→ Fable.Transforms + Fable.Compiler) and Fable.Standalone build clean.
• No reference-identity / null / Unchecked.defaultof usages on the changed compiler-logic types (the only behavioral axis [<Struct>] affects).
• JavaScript quicktest compiles and runs end-to-end with the freshly built compiler.

🤖 Generated with Claude Code

[github-actions]

Python Type Checking Results (Pyright)

Metric	Value
Total errors	34
Files with errors	4
Excluded files	4
New errors	✅ No

Excluded files with errors (4 files)

These files have known type errors and are excluded from CI. Remove from pyrightconfig.ci.json as errors are fixed.

File	Errors	Status
temp/tests/Python/test_hash_set.py	18	Excluded
temp/tests/Python/test_applicative.py	12	Excluded
temp/tests/Python/test_nested_and_recursive_pattern.py	2	Excluded
temp/tests/Python/fable_modules/thoth_json_python/encode.py	2	Excluded