Back ordered dictionaries `[K: V]` with IndexMap (codegen + lifting)

[antoniusnaumann]

Spec (Galvan by Example)

• collections/index.md: [K: V] (ordered dictionary) maps to IndexMap<K, V> — the "currently BTreeMap" caveat was removed.
• interop/liftings.md: Rust IndexMap<K, V> lifts to [K: V]; BTreeMap is no longer in the lifting table.

Ordered = insertion order (IndexMap), not key order (BTreeMap).

Current behavior

• Codegen emits ::std::collections::BTreeMap for both the type (galvan-transpiler/src/transpile_item/type.rs:17-22) and literals (codegen/expression.rs:738-739); indexmap is not a dependency anywhere.
• Lifting is inverted vs the book: classify_wrapper lifts BTreeMap → ordered dict and leaves IndexMap nominal (galvan-rustdoc/src/interop/lift_wrappers.rs:150-174).

Implementation sketch

• Add indexmap to the runtime crate (root Cargo.toml) and re-export from src/std/mod.rs
• Type lowering → ::galvan::std::IndexMap<K, V>; literal lowering → ::galvan::std::IndexMap::from([...])
• Lifting swap in lift_wrappers.rs: indexmap::IndexMap (path-aware, like the std checks) → ordered dict; BTreeMap falls through to a nominal imported type
• Update galvan-rustdoc/LIFTINGS.md (the authoritative contract) and its tests

Watch out

Iteration order changes from sorted to insertion order — audit existing ordered-dict e2e tests/examples for implicit sorted-order assumptions.

Docs

Remove the BTreeMap [!WARNING] in collections/ordered_dictionaries.md and regenerate its Generated Rust blocks.

Part of aligning the implementation with the Galvan by Example book (the book is the spec).