Fix ContextExtension serialization order

[Luivatra]

Root Cause: Issue #763

When a ContextExtension has 5+ entries, the Ergo node (Scala 2.12) serializes them in HAMT hash-based order, not sorted/insertion order. sigma-rust used insertion order for all sizes, causing bytes_to_sign to differ, which makes the TxId different,
which invalidates the proof signature.

Threshold behavior:

• 1-4 entries: Scala uses Map1-Map4 (insertion order) — matches sigma-rust
• 5+ entries: Scala uses HashMap (HAMT iteration order) — diverges from sigma-rust

The Fix

Single file modified: ergotree-ir/src/chain/context_extension.rs

What was added:

1. scala_212_improve(hc: i32) -> i32 — Replicates Scala 2.12's hash improvement function used by its HAMT
2. scala_212_hamt_sort_key(key: u8) -> u64 — Computes a sort key that matches HAMT iteration order (encodes 5-bit slots at each trie level)
3. Modified sigma_serialize — For 5+ entries, sorts by HAMT slot order before writing; for <5, preserves insertion order (unchanged behavior)

Tests added (4):

• test_scala_212_improve — Verifies level-0 HAMT slots match empirically observed node behavior
• test_hamt_sort_order_6_entries — Keys {0,1,2,3,4,5} sort to [0,5,1,2,3,4] matching the node
• test_serialize_order_5plus_entries — Verifies serialized bytes have keys in HAMT order
• test_serialize_order_4_entries_unchanged — Verifies <5 entries still use insertion order