LLM Oneshot Prompts, Oneshotted Apps, and Cursor Rules (C#/Rust/TS) (#4032)

# Description of Changes

This PR introduces an **LLM One-Shot App Generation** benchmarking
framework and comprehensive **Cursor rules for SpacetimeDB
development**.

**Cursor Rules (`docs/.cursor/rules/`):**
- `spacetimedb.mdc` - General SpacetimeDB concepts and architecture
- `spacetimedb-csharp.mdc` - C# module and client patterns
- `spacetimedb-rust.mdc` - Rust module development
- `spacetimedb-typescript.mdc` - TypeScript SDK usage

**LLM One-Shot Framework (`tools/llm-oneshot/`):**
- Benchmarking tool to measure how well AI can one-shot SpacetimeDB apps
- Composable prompt system with 12 cumulative feature levels (basic chat
→ full-featured with threading, permissions, presence, etc.)
- Support for 4 tech stacks: TypeScript+SpacetimeDB,
TypeScript+PostgreSQL, Rust+SpacetimeDB, C#+SpacetimeDB
- Additional Cursor rules for deployment, grading, and frontend patterns

**Sample One-Shotted Apps:**
- Multiple chat-app implementations (TypeScript, Rust, C#/MAUI)
- Multiple paint-app implementations (TypeScript, Rust, C#/MAUI)

# API and ABI breaking changes

None - this is a documentation and tooling addition only.

# Expected complexity level and risk

**1** - Trivial addition of documentation, tooling, and example
applications. No changes to core SpacetimeDB functionality. The
`tools/llm-oneshot/` folder is entirely self-contained and the Cursor
rules in `docs/` are purely informational.

# Testing

- [ ] Verified Cursor rules load correctly in IDE
- [ ] Ran one-shot generation with various prompt levels to validate
rules work
- [ ] Sample apps compile and deploy correctly

Author: bradleyshep