Make --preserve-order a global CLI flag instead of per-function parameter

[vivainio]

Summary

• Replace the exp-preserve-order feature flag and per-function preserve_order parameters with a global thread-local setting (following the existing STACK_LIMIT pattern)
• The --preserve-order CLI flag sets this globally, affecting all object field iteration without polluting stdlib function signatures
• Net deletion of ~400 lines of #[cfg]-gated code and parameter threading

Motivation

Feedback on #217: adding preserve_order to every stdlib function pollutes the API surface. A global flag is cleaner — users who want preserved order want it everywhere, not per-call.

How it works

• Thread-local PRESERVE_ORDER: Cell<bool> in obj/mod.rs with set_preserve_order()/preserve_order() accessors
• ObjValue::fields(), iter(), etc. read the global setting instead of taking a parameter
• CLI sets set_preserve_order(true) at startup when --preserve-order is passed
• The real ordering module (with FieldIndex/SuperDepth tracking) is always compiled — no more no-op stub

Test plan

• cargo build succeeds
• cargo test passes
• jrsonnet -e '{ c: 1, b: 2, a: 3 }' outputs alphabetical order
• jrsonnet --preserve-order -e '{ c: 1, b: 2, a: 3 }' preserves declaration order