[CI] Contracts CI optimizes WASM but enforces no size budget (no wasm-size gate)

[grantfox-oss]

Telegram (ask questions / claim the issue here first): https://t.me/+DOylgFv1jyJlNzM0

Why this matters

ci.yml builds and optimizes the contract WASM and uploads it as an artifact, and the release profile is tuned for size (opt-level=z, lto, strip). But there is no step that asserts the optimized .wasm stays under a size budget, so a size regression (which directly affects Soroban deploy/instantiation cost) merges unnoticed.

Acceptance criteria

• After the optimize step, fail the job if the optimized .wasm exceeds a configured byte budget
• Print the optimized size to the job log for visibility
• Document the chosen budget

Files to touch

• .github/workflows/ci.yml
• contracts/Cargo.toml

Out of scope

• Gas/CPU instruction budgeting