docs(contracts): add reproducible deploy and upgrade scripts (#40)

[abrak01]

Problem

No baseline existed for contract CPU cost. Each Soroban host call (address
clone, storage read/write) is metered — redundant ones add up across every user
transaction.

Changes

Three files touched, one focused optimization per contract:

vinculo_sbt/src/lib.rs

• mint: replaced has() + get() for the admin check with a single get() — saves
  one instance-storage read per mint.
• mint / revoke: build DataKey::Tier(user) once instead of cloning user before
  each storage call.

staking_pool/src/lib.rs

• stake: build DataKey::Stake and DataKey::Balance keys once — was cloning user
  6 times.
• unstake: same key reuse (4 clones removed); replaced field-by-field struct
  zeroing with StakeInfo::default() (4 field writes → 1).

vinculo_lending/src/lib.rs

• request_loan: reuse the instance() storage reference for both Token and Sbt
  reads; build DataKey::Loan(user) once instead of 3 times.
• repay: same loan key reuse.

Benchmark

Each contract now includes bench_* tests that print cpu_instruction_cost and
memory_bytes_cost via env.budget():

run from any contract directory

cargo test --features soroban-sdk/testutils -- --nocapture

Behavior unchanged

Pure mechanical refactor — same logic, same storage keys, same return values.
All existing tests are preserved.

Reviewer step

Run the benchmark command above and confirm all tests pass and benchmark lines
print.

Closes #41

[vercel]

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