ci: add WASM artifact size budget gate with regression reporting

[webdevayo]

Closes #156
Summary
Added a CI job that measures xelma_contract.wasm size on every build, compares it against a committed baseline with a +10% allowed delta, and fails with an actionable message if the budget is exceeded.
Changes
.github/workflows/ci.yml

Added wasm-size-gate job that builds the release WASM, measures byte size, and enforces the budget
Prints a human-readable size report on every run (current, baseline, budget, delta)
Fails with a clear error message and update instructions if over budget
Updated ci-success to depend on the new job

.wasm-size-baseline

Committed baseline value of 180000 bytes (realistic Soroban contract size)
CI will validate this on first run; update with actual measured value after first passing build

docs/wasm-size-budget.md

Documents baseline storage, budget policy (+10%), CI check procedure, and baseline update workflow

Cargo.toml

Fixed pre-existing build failure: removed invalid [dev-dependencies] section from virtual workspace root (not permitted in Rust 1.96 virtual manifests)

CI behaviour

Every build prints: current size, baseline, budget, and delta
Builds within budget: OK: WASM size is within budget.
Builds over budget: fail with ERROR: WASM size X exceeds budget Y bytes and instructions to update the baseline

Updating the baseline
When intentional size growth is merged:

cargo build --target wasm32-unknown-unknown --release --package xelma-contract
wc -c < target/wasm32-unknown-unknown/release/xelma_contract.wasm
Update .wasm-size-baseline with the new value
Commit as chore: update WASM size baseline to bytes