fix(contracts): align Cargo workspace members and document criteria (#1002)

[devpassionOX]

Overview

This PR aligns the contracts/ Cargo workspace with every buildable Soroban contract crate, documents clear membership criteria, and adds a cargo check --workspace CI step so contributors can tell workspace packages from educational templates and incomplete scaffolding.

Related Issue

Closes #1002

Changes

📦 Workspace Alignment

• [MODIFY] contracts/Cargo.toml

  • Adds all remaining buildable contract crates as workspace members (alphabetically sorted).
  • Excludes incomplete scaffolding such as did_registry/ (no Cargo.toml yet).
  • Removes nested empty [workspace] tables from previously standalone crates so they join the root workspace cleanly.

• [ADD] docs/contracts/WORKSPACE.md

  • Documents selection criteria for workspace vs non-workspace content.
  • Clarifies that contracts/src/*.rs educational templates belong to the root package and are not separate members.
  • Provides contributor steps for adding a new contract crate.

• [MODIFY] .github/workflows/ci.yml

  • Runs cargo check --workspace, cargo clippy --workspace, and cargo build --workspace.

🔧 Compile Fixes For Newly Covered Members

• [MODIFY] contracts/payment_gateway/src/lib.rs — replace invalid assert!(..., Error) usage, fix ledger timestamp typing, shorten event symbols to ≤9 chars.
• [MODIFY] contracts/zk_proof_verifier/src/lib.rs — convert Hash<32> via .into() and hash addresses with ToXdr.
• [MODIFY] contracts/hello_world/src/lib.rs — accept String greeting input so the crate builds under no_std.

Verification Results

cd contracts
cargo check --workspace
✅ Finished `dev` profile (exit 0)

Membership coverage check:
✅ 26/26 directories with Cargo.toml are workspace members
✅ Incomplete `did_registry/` excluded (documented)

Acceptance Criteria	Status
Document selection criteria for workspace members	✅ docs/contracts/WORKSPACE.md
Add all buildable contracts to the workspace (or organize sub-workspaces)	✅ All compiling crates added; incomplete dirs excluded with rationale
Add cargo check --workspace CI validation	✅ Contracts CI job updated

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