Add worst-case critbit tree depth tests to order book

[mikemaccana]

Summary

A critbit tree does not rebalance. Asks at doubling prices each set a new highest price bit, so each one adds a level to the path to the best ask. The price fills the top 64 bits of the 128-bit key, so prices can add at most 64 levels, and no path can exceed 128. This adds tests that build that worst case and measure what it costs, in both Anchor copies.

Tests (Anchor v2 and Anchor v1)

• doubling_prices_build_the_deepest_path_prices_allow: four funded sellers place 63 asks at 2^1 through 2^63 (at most 20 open orders each). Two probe asks then go in at price 1. The test reads the asks tree from the account's bytes and asserts that the best ask is 64 inner nodes deep, and 1 deep on a book holding only the probes.
• deepest_path_adds_little_compute_to_insert_fill_and_cancel: runs the same insert, fill, and cancel at the bottom of that path and on a shallow book. It asserts that each costs under 15,000 extra compute units and fits the default 200,000-unit instruction budget.

Measured, in compute units:

	v2 shallow	v2 depth 64	v1 shallow	v1 depth 64
insert	13,715	24,253	26,612	38,754
fill	24,380	35,444	31,283	41,941
cancel	2,816	10,844	8,925	16,871

Other changes

• The v1 copy gains a solana-transaction dev-dependency, at the version solana-kite already builds against, so the test can read each transaction's compute units. solana-kite's send helper doesn't return them.
• Both READMEs no longer say the tree "stays shallow no matter what order keys arrive in", or that Phoenix uses the same structure. Phoenix uses a red-black tree. The expected test output there was stale and is brought up to date.
• CHANGELOG entries at the root and in both copies.

Compatibility with #158 (eviction)

I merged this branch onto #158's head in a scratch worktree and ran the v2 suite: 34 of 34 pass. With eviction, the deep fill costs 13,736 extra units, still under the 15,000 limit. The textual conflicts in the tests, READMEs, and CHANGELOGs come from both branches appending to the same files. Whichever of the two merges second needs to keep both sides.

cargo test passes in finance/order-book/anchor and finance/order-book/anchor-v1: 29 of 29 each.

🤖 Generated with Claude Code

https://claude.ai/code/session_01RQ5RZgoQjVK36qSjqUSq6X