Docs 798 stream contract readme

[Killerjunior]

Description
Add dedicated crate-level documentation for contracts/stream_contract so integrators have a single entry point covering the public API, lifecycle, fee/treasury model, event topics, and error codes.

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• 🔧 Refactoring (no functional changes)
• ⚡ Performance improvement
• 🧪 Test addition or update

Related Issues

Closes #798

Changes Made

• Added contracts/stream_contract/README.md with:
  • contract overview and stream lifecycle (create/top_up/withdraw/cancel/pause/resume)
  • protocol fee and treasury model details
  • explicit note on fee rounding-to-zero behavior
  • full event topic reference
  • StreamError code/variant table
• Added package description to contracts/stream_contract/Cargo.toml.
• Wired crate docs to README via #![doc = include_str!("../README.md")] in contracts/stream_contract/src/lib.rs so generated docs and source of truth stay aligned.

Testing

Test Coverage

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed

Test Steps

1. Open contracts/stream_contract/README.md and verify API, fee model, events, and error table are present.
2. Verify contracts/stream_contract/src/lib.rs includes #![doc = include_str!("../README.md")].
3. Verify contracts/stream_contract/Cargo.toml includes a description field.

Breaking Changes

Breaking Changes:

• None.

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published
• I have checked for breaking changes and documented them if applicable