feat(farming-pool): support partial unstake (#14)

[storm-beyndtech]

Closes #14.

What

unstake() previously withdrew the entire staked balance and always
removed the stake record — unlike unlock_assets(), which already supports
partial withdrawal via an amount parameter. This makes the stake/boost path
symmetric with the lock/unlock path.

Changes

• Add amount: i128 to unstake(env, from, amount).
• Validate amount > 0 → PoolError::InvalidAmount and
  amount <= stake.amount → PoolError::InsufficientStake, returning typed
  errors instead of panicking (two new PoolError variants added).
• checkpoint before mutating amount, so credits accrue against the full
  staked balance up to the unstake ledger.
• Remove the stake on full exit; persist the reduced stake on partial exit.
• Emit (pool, unstaked) → (user, amount, credits_snapshot).
• Drive-by: upgraded the missing-stake path from .expect("no active stake")
  to a typed PoolError::NoActiveStake (the variant already existed).

Tests (all green — cargo test --workspace, 79 passed)

• partial unstake preserves remaining balance + banked credits
• full unstake clears storage and returns all tokens
• insufficient balance → typed InsufficientStake
• non-positive amount (zero/negative) → typed InvalidAmount
• event emission
• existing unstake call sites updated for the new signature

Verified the release WASM still builds: cargo build --target wasm32v1-none --release.

Decisions / open questions

• Return value on partial unstake: followed the spec and return
  credits_snapshot (banked credits at unstake time) on both partial and full
  exit. Trivial to flip to "return only on full exit" if you'd prefer that.
• Typed errors vs. asserts: used typed PoolErrors for validation per the
  acceptance criteria. Note unlock_assets still uses assert! for its amount
  checks — I kept this change scoped and didn't refactor it; happy to align both
  separately.
• Event API: used env.events().publish(...) to match every other event in
  the contract. The SDK now deprecates this in favor of #[contractevent] —
  happy to migrate the whole file in a separate PR rather than make unstake the
  odd one out here.

Note (out of scope, possible follow-up issue)

make build targets wasm32-unknown-unknown, which soroban-sdk 25.3.1 rejects
outright (requires wasm32v1-none, which is what CI uses). The Makefile's build
target is broken against the current SDK — flagging it for a separate fix.