Add fuzz testing intro with link to Rendezvous docs

This includes:
- Section README introduction
- Fuzz testing guide with key concepts and tool overview
- Link to Rendezvous documentation
- SUMMARY.md updates for navigation

Author: moodmosaic

SUMMARY.md

@@ -74,6 +74,8 @@
   * [Post Conditions with Stacks.js](guides-and-tutorials/frontend/post-conditions-with-stacks.js.md)
   * [Authentication with Stacks.js](guides-and-tutorials/frontend/authentication-with-stacks.js.md)
   * [Sending Transactions with Stacks.js](guides-and-tutorials/frontend/sending-transactions-with-stacks.js.md)
+* [Testing Smart Contracts](guides-and-tutorials/testing-smart-contracts/README.md)
+  * [Fuzz Testing](guides-and-tutorials/testing-smart-contracts/fuzz-testing.md)
 * [Run a Node](guides-and-tutorials/nodes-and-miners/README.md)
   * [Run a Node with Docker](guides-and-tutorials/nodes-and-miners/run-a-node-with-docker.md)
   * [Run a Node with Digital Ocean](guides-and-tutorials/nodes-and-miners/run-a-node-with-digital-ocean.md)

guides-and-tutorials/testing-smart-contracts/README.md

@@ -0,0 +1,11 @@
+# Testing Smart Contracts
+
+Smart contracts are immutable once deployed. Bugs are permanent. Test them
+thoroughly.
+
+This section covers testing Clarity contracts.
+
+- **Fuzz Testing**: Use Rendezvous to hammer your contract with random
+  inputs. It helps expose edge cases and vulnerabilities.
+
+More guides will follow.

guides-and-tutorials/testing-smart-contracts/fuzz-testing.md

@@ -0,0 +1,55 @@
+# Fuzz Testing with Rendezvous
+
+Smart contracts on Stacks are immutable. Bugs are forever. Test early. Test
+often. Fuzzing finds edge cases that unit tests often miss.
+
+## What is Fuzz Testing?
+
+Fuzzing hits your code with random inputs. It helps uncover unexpected
+behavior and subtle bugs. Unlike unit tests, it explores paths you didn't
+think of.
+
+Rendezvous (`rv`) is a Clarity fuzzer. It supports:
+
+### Property-Based Testing
+
+You define a property. Rendezvous checks it with random inputs.
+
+- **Properties**: Truths that must always hold.
+- **Generators**: Create random input data.
+- **Shrinking**: Reduce failing input to the minimal case.
+
+### Invariant Testing
+
+You define conditions that must always hold. Rendezvous runs sequences of
+calls to verify them.
+
+- **Invariants**: Must always be true.
+- **State transitions**: What changes with each call.
+- **Function sequences**: Random sequences of calls to test state consistency.
+
+## Why Test in Clarity?
+
+Rendezvous tests run in Clarity, just like your contracts.
+
+1. Tests operate under the exact same constraints as production code.
+2. Better understanding of Clarity.
+3. No need to expose internals as public functions.
+4. Fewer tools to manage.
+
+## Getting Started
+
+Put tests next to contracts. Rendezvous will find them.
+
+```
+my-project/
+├── Clarinet.toml
+├── contracts/
+│   ├── my-contract.clar       # Contract
+│   ├── my-contract.tests.clar # Tests
+└── settings/
+    └── Devnet.toml
+```
+
+See full docs at:
+https://stacks-network.github.io/rendezvous/