feat: perf: optimize contract storage access patterns

[Mirabliss]

Audit and fix three redundant/expensive storage reads:

1. join_group: remove redundant GroupStatus SLOAD

   • Previously loaded group.status via a separate status_key SLOAD
   • Now reads group.status from the already-loaded Group struct
   • Saves 1 persistent SLOAD per join_group call

2. contribute: move reentrancy guard to temporary storage

   • Temporary storage is ~10x cheaper than persistent on Soroban
   • Guard is auto-cleared at ledger close; no persistent slot needed
   • Saves ~3 persistent storage ops per contribute call

3. execute_payout: move reentrancy guard to temporary storage

   • Same fix as contribute() — guard moved from persistent to temporary
   • Saves ~2 persistent storage ops per payout call

4. gas_benchmark: update op counts and assertions to reflect new savings

   • contribute() persistent ops: 19 before -> 13 after (31% reduction)
   • Benchmark table updated with accurate post-optimization counts

Summary

Motivation

Closes #

Type of change

• feat — new feature
• fix — bug fix
• docs — documentation only
• refactor — code restructuring, no behaviour change
• test — tests only
• chore — build, deps, tooling
• perf — performance improvement
• Breaking change (existing behaviour changes)

Changes

Testing

Smart contract:

cargo test -p stellar-save

Frontend:

cd frontend && npm test run

Manual steps (if applicable):

Checklist

• cargo fmt run (Rust changes)
• cargo clippy -- -D warnings passes (Rust changes)
• npm run lint passes (frontend changes)
• Tests added or updated for new/changed behaviour
• CI is green
• Documentation updated (if behaviour changed)
• No secrets or .env files staged

Screenshots / recordings

closes #894

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