Stabilize --preserve-order as a non-experimental CLI flag

[vivainio]

Summary

• Remove the exp-preserve-order compile-time feature gate across all crates
• Make --preserve-order CLI flag and preserve_order parameter on std.manifest*/std.objectFields* always available
• Add test coverage for preserve-order behavior

Implements #212

Details

The preserve_order functionality was already fully implemented behind the exp-preserve-order feature. This PR simply removes the #[cfg(feature = "exp-preserve-order")] gates from all 28 affected files, making the feature unconditionally available while keeping it opt-in (not enabled by default).

No behavioral changes — the same code that ran with --features exp-preserve-order now runs in all builds.

Test plan

• cargo check passes
• All workspace tests pass
• std_param_names test updated for new parameter signatures
• New preserve_order.jsonnet test covering: field ordering, objectFields/Values/KeysValues, manifestJson output, field override behavior, and object comprehension order

[CertainLach]

I don't want to enable it by default unless the behavior is decided on cpp-jsonnet/go-jsonnet side

[vivainio]

I don't want to enable it by default unless the behavior is decided on cpp-jsonnet/go-jsonnet side

It would be a flag that is disabled by default

[CertainLach]

I'm more concerned about preserve_order argument of stdlib functions

I'll think about it.

[vivainio]

I agree that adding preserve_order to every stdlib function is not the right approach. I've opened an alternative PR #218 that makes --preserve-order a global-only CLI flag instead — no changes to stdlib function signatures at all. It uses a thread-local (same pattern as STACK_LIMIT) that all object iteration reads from automatically.

[CertainLach]

I agree that adding preserve_order to every stdlib function is not the right approach

This is the right approach, as having that as a global flag braks many usecases with internal manifestification, I only meant that preserve_order argument on stdlib functions should not be stabilized until consensus is reached with jsonnet upstream

[vivainio]

Do you have a particular use case that would break in mind? Global flag is all-or-nothing, and without it everything would behave as before