fix(soroban): extract escrow_contract into its own crate to fix wasm build

[Meshmulla]

Summary

Second follow-up to #13 (alongside #14). With the ethnum fix in #14 applied, the wasm build got further but then failed with:

error: symbol `initialize` is already defined
   --> soroban/src/escrow_contract.rs:112:1

Root cause

soroban/src/lib.rs's own top-level contract and escrow_contract.rs's TimeLockedEscrowContract both export a wasm symbol named initialize. Since both compile into the same cdylib, their wasm export tables collide. There's already a comment in lib.rs explaining that governance and insurance_pool were previously made test-only (#[cfg(test)]) for exactly this reason — escrow_contract just never got the same treatment and was still being compiled unconditionally into the release wasm artifact.

I checked all 18 #[contract]-bearing modules in the crate for method-name collisions; restricting to the ones that are actually compiled unconditionally (i.e. not already behind #[cfg(test)]), the only collision is this one: initialize between lib.rs and escrow_contract.rs.

Fix

Extract escrow_contract into its own workspace crate (escrow_contract/), matching the existing transfer_state_machine/ layout — its own Cargo.toml, its own wasm cdylib output. The module had zero crate:: dependencies on the rest of soroban, so this was a straight relocation:

• soroban/src/lib.rs: drop pub mod escrow_contract;
• soroban/Cargo.toml: drop the escrow_contract_test [[test]] entry
• root Cargo.toml: add escrow_contract to workspace members
• Cargo.lock: add the escrow-contract workspace member entry
• README.md: document the new crate in the workspace table
• escrow_contract/src/lib.rs: add #![no_std] at the crate root (it previously inherited this from soroban/src/lib.rs as a submodule; as its own crate root it needs the attribute itself, otherwise linking both std's and soroban-sdk's panic handler duplicates the panic_impl lang item under wasm32-unknown-unknown — E0152)
• Fixed a stale import in the moved test file that referenced the crate's pre-rename name bridge_watch_contracts instead of the current escrow_contract

Testing/validation performed

Pushed this change combined with #14's ethnum bump to a disposable branch and confirmed via a real GitHub Actions run that cargo build --release --target wasm32-unknown-unknown now succeeds for the full workspace (soroban, transfer_state_machine, and the new escrow_contract crate all compile). That disposable branch/run has been deleted — it was validation only, not a deliverable.

Note: cargo test still fails on main for reasons unrelated to this PR or #14 (several pre-existing, independent issues: stale bridge_watch_soroban crate-name references in a couple of test files, a test file missing a [[test]] entry causing an invalid auto-discovered target name, a field/method mismatch against the current soroban-sdk version, and a format! macro resolution issue under no_std). Those are out of scope here since they're unrelated to the wasm build collision this PR fixes — flagging for a separate follow-up.

Related

• Depends on / pairs with fix(deps): bump ethnum 1.5.2 -> 1.5.3 to fix build on current Rust stable #14 (ethnum bump) — both are needed together for cargo build --release --target wasm32-unknown-unknown to succeed.
• Follow-up to feat(ci): add CI workflow to build, test, and validate contracts #13.