feat: wasm build size

[henrypeters]

What was implemented

.github/workflows/wasm-size.yml (new, 109 lines)
A dedicated workflow that triggers on any contract/** change. It:

• Installs the stable Rust toolchain with wasm32-unknown-unknown target
• Runs cargo build --release --target wasm32-unknown-unknown
• Generates wasm-size-report.txt with per-contract byte count, KiB, and a total row
• Uploads it as the wasm-size-report artifact (retained 30 days) — this is the downloadable CI artifact
• Writes a Markdown table to the GitHub step summary for inline visibility

.github/workflows/ci.yml (modified)
Added a wasm-size job that runs after the existing contract job (needs: contract). It reuses the same Cargo cache keys so the Wasm build is fast (registry already
warm), then generates the same report and uploads the same artifact.

.github/workflows/contract-release.yml (modified)
The existing 🏗️ Build WASM release artifacts step already built Wasm but discarded the output. Two steps were inserted immediately after it:

• 📏 Generate Wasm size report — writes wasm-size-report.txt
• 📦 Upload Wasm size report artifact — uploads it with if-no-files-found: error so a missing report fails the job

Manual checklist to verify:

1. Open any PR touching contract/ → CI runs → "Wasm Build Size" job appears
2. Click the job → step summary shows the size table
3. Go to the run's "Artifacts" section → download wasm-size-report → open wasm-size-report.txt and confirm per-contract sizes and total are present

closes #622

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