Add price accumulator support for TWAP oracle

[0x-r4bbit]

The TWAP oracle program (RFP-019) depends on the AMM pool exposing a price accumulator — a running record of the pool's price over time.
Currently PoolDefinition only stores the current reserves. This issue tracks the changes needed in the AMM program so RFP-019 has the data
it needs.

Background

A TWAP (Time-Weighted Average Price) is computed from two accumulator snapshots:

twap_tick = (tickCumulative[t2] - tickCumulative[t1]) / (t2 - t1)

tickCumulative is a running sum of current_tick × elapsed_seconds, updated at block boundaries before any trades execute. The current_tick is an integer encoding of the current spot price derived from the reserve ratio:

tick = floor( log(reserve_b / reserve_a) / log(1.0001) )

Computed via integer bit manipulation (MSB + iterative squaring) — no floating point.

What needs to be done

1. Add accumulator fields to PoolDefinition in amm_core

pub struct PoolDefinition {
    // existing fields ...

    /// Current price tick, derived from reserve_b / reserve_a.
    /// Updated on every state-changing operation.
    pub current_tick: i32,

    /// Timestamp (ms) at which current_tick was last recorded.
    /// Used by the oracle program to advance tickCumulative.
    pub last_observation_timestamp: u64,
}

2. Compute and write current_tick on every state change

Every instruction that modifies reserves must update current_tick and last_observation_timestamp before writing the pool post-state.
The tick must be computed from the post-swap reserves (the price after the trade, not before), because that is what the next block's
accumulator advance will be based on.

3. Implement tick_from_reserves(reserve_a, reserve_b) -> i32

Pure integer implementation in amm_core. No floating point. Approach:

1. Represent price = reserve_b / reserve_a as a fixed-point integer
2. Find floor(log2(price)) via 127 - leading_zeros() on the numerator
3. Refine to the tick index using iterative squaring against the log(1.0001) constant (Uniswap v3 TickMath approach)

Acceptance criteria

• PoolDefinition contains current_tick: i32 and last_observation_timestamp: u64
• tick_from_reserves() is implemented in amm_core with unit tests covering known reserve ratios → expected ticks
• All state-changing instructions update both fields in their post-state output
• Existing tests remain green
• New tests cover: tick computed correctly after NewDefinition, tick updated after a swap that changes the reserve ratio