Restructure into a Cargo workspace (prep for factory/router/LP-token contracts)

[Jaydbrown]

Summary

First of a series of PRs to partition this repo into multiple contracts (factory, standalone LP token, router) instead of the single monolithic NodusAmm pool contract it has today. This PR is purely structural — no contract behavior changes — and lays the groundwork the others depend on.

• Moves the existing crate to contracts/pool/, adds a workspace root Cargo.toml.
• [profile.release] moved to the workspace root (Cargo only honors it there for members); soroban-sdk's version is now declared once via [workspace.dependencies] so every future contract crate stays in lockstep.
• The workspace's shared target/ dir means the WASM artifact still lands at the exact same path as before (target/wasm32v1-none/release/nodus_protocol_amm.wasm), and every existing cargo build/test/clippy/fmt invocation still works unmodified from the repo root — verified locally.
• CI: cargo fmt -- --check → cargo fmt --all -- --check. Both currently behave identically with only one workspace member, but bare cargo fmt isn't guaranteed to cover every member as more contracts are added, and this also matches what the Makefile's lint target already did.
• README: updated the repo-structure diagram for the new path, and fixed two stale references left over from an earlier crate rename (wasm32-unknown-unknown → wasm32v1-none, nodus_amm.wasm → nodus_protocol_amm.wasm) — noticed while editing the same section, not otherwise related to this PR.

Test plan

• cargo build --release --target wasm32v1-none — succeeds, same output path
• cargo test --features testutils — 52 passed, 0 failed (unchanged)
• cargo clippy --all-targets --features testutils -- -D warnings — clean
• cargo fmt --all -- --check — clean

Note on account access

Opened from my personal fork — my nodus-protocol org access was revoked partway through this work (confirmed via gh api repos/.../permissions). Same situation as the mobile-repo PRs from earlier today.

What's next

• PR 2: extract LP tokens out of the pool into their own standalone contract, with the pool calling it cross-contract for mint/burn.
• PR 3: Factory contract (deploys + tracks a pool per token pair, replacing the current one-hardcoded-pair setup).
• PR 4: Router contract (multi-hop swaps, convenience add/remove-liquidity across pools).