refactor(all): add [<return: Struct>] to partial active patterns

[dbrattli]

Summary

Resolves all open IONIDE-009 code scanning alerts ("Consider adding [<return: Struct>] to partial active pattern") in src/Fable.Transforms. Converts 73 partial active patterns to struct-returning (ValueOption) form across 9 files.

Struct active patterns avoid the heap allocation of Option on every pattern match, which matters in the hot compiler paths these patterns sit on (replacements, AST transforms).

What changed

For each flagged pattern: added [<return: Struct>] and changed only the return-position values to ValueSome/ValueNone. Nested option match patterns (e.g. { FieldType = Some t }), inner option plumbing, and cross-pattern/recursive calls in pattern position were left untouched. Where a pattern's result came from a helper returning a plain option, it is bridged with an explicit match or ValueOption.ofOption.

File	Patterns
FSharp2Fable.Util.fs	19
Replacements.Util.fs	21
Transforms.Util.fs	14
Replacements.fs	5
Global/Prelude.fs	4
Fable2Babel.fs	4
Global/Naming.fs	3
FableTransforms.fs	2
Python/Replacements.fs	1

(|CustomOp|_|) is also invoked as a plain function (not in pattern position) at two sites in Beam/Replacements.fs; those matches are adapted to the new voption result.

Intentionally skipped

Tags.(|Contains|_|) at src/Fable.AST/Fable.fs:12 (1 alert) is not changed — Fable.AST is the public AST and switching an active pattern's return type from Option to ValueOption is a binary-breaking change requiring a major version bump (per AGENTS.md).

Verification

dotnet build src/Fable.Cli/Fable.Cli.fsproj -c Release (covers all targets — JS/TS, Python, Rust, Dart, BEAM) succeeds with 0 warnings, 0 errors.

🤖 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