docs: document defi-wonderland immutable macro (#21764)

## Summary
- Adds documentation for the
[aztec-immutables-macro](https://github.com/defi-wonderland/aztec-immutables-macro/tree/dev)
library that encodes immutable values into the contract deployment salt,
eliminating initialization transactions
- Adds the page to both current developer docs and v4.1.0-rc.2 versioned
docs

## Test plan
- [ ] Verify page renders correctly with `yarn start` in docs/
- [ ] Confirm the page appears in the v4.1.0-rc.2 version dropdown under
Aztec.nr > Framework Description
- [ ] Verify external links to the macro repo are valid

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Author: critesjosh

docs/developer_versioned_docs/version-v4.1.0-rc.2/docs/aztec-nr/framework-description/immutables.md

@@ -0,0 +1,33 @@
+---
+title: Immutables via Salt
+sidebar_position: 4
+tags: [contracts, storage, immutables, optimization]
+description: Commit immutable values into a contract's address salt, eliminating initialization transactions.
+---
+
+Aztec contracts can commit immutable values directly into the contract's address by encoding them into the deployment salt, removing the need for a separate initialization transaction.
+
+## Overview
+
+Rather than storing immutables in private storage (which requires an initializer function and an extra transaction), the [aztec-immutables-macro](https://github.com/defi-wonderland/aztec-immutables-macro/tree/dev) library encodes them into the contract's salt:
+
+```
+salt = poseidon2_hash([actual_salt, constant_0, constant_1, ...])
+```
+
+Since the salt is part of the address derivation, the immutable values become cryptographically bound to the contract's address itself.
+
+## Key benefits
+
+- **No initialization transaction** — immutables are committed at deployment time, not in a separate setup call
+- **Runtime verification** — at execution time, capsule data is loaded and verified against the stored salt, ensuring data integrity
+- **Persistent storage** — immutables are persisted to the PXE's [CapsuleStore](./advanced/how_to_use_capsules.md) after deployment, so capsules don't need to be attached to every transaction
+- **Compatible with standard storage** — works alongside `#[storage]` and initializers when needed
+
+## Performance
+
+Initialization cost is completely eliminated (no constructor transaction). The per-transaction overhead is approximately 1,098 gates (+0.2%) in the account entrypoint.
+
+## Getting started
+
+For installation instructions, usage examples, and a reference implementation of an initializerless Schnorr account contract, see the [aztec-immutables-macro README](https://github.com/defi-wonderland/aztec-immutables-macro/tree/dev).

docs/docs-developers/docs/aztec-nr/framework-description/immutables.md

@@ -0,0 +1,33 @@
+---
+title: Immutables via Salt
+sidebar_position: 4
+tags: [contracts, storage, immutables, optimization]
+description: Commit immutable values into a contract's address salt, eliminating initialization transactions.
+---
+
+Aztec contracts can commit immutable values directly into the contract's address by encoding them into the deployment salt, removing the need for a separate initialization transaction.
+
+## Overview
+
+Rather than storing immutables in private storage (which requires an initializer function and an extra transaction), the [aztec-immutables-macro](https://github.com/defi-wonderland/aztec-immutables-macro/tree/dev) library encodes them into the contract's salt:
+
+```
+salt = poseidon2_hash([actual_salt, constant_0, constant_1, ...])
+```
+
+Since the salt is part of the address derivation, the immutable values become cryptographically bound to the contract's address itself.
+
+## Key benefits
+
+- **No initialization transaction** — immutables are committed at deployment time, not in a separate setup call
+- **Runtime verification** — at execution time, capsule data is loaded and verified against the stored salt, ensuring data integrity
+- **Persistent storage** — immutables are persisted to the PXE's [CapsuleStore](./advanced/how_to_use_capsules.md) after deployment, so capsules don't need to be attached to every transaction
+- **Compatible with standard storage** — works alongside `#[storage]` and initializers when needed
+
+## Performance
+
+Initialization cost is completely eliminated (no constructor transaction). The per-transaction overhead is approximately 1,098 gates (+0.2%) in the account entrypoint.
+
+## Getting started
+
+For installation instructions, usage examples, and a reference implementation of an initializerless Schnorr account contract, see the [aztec-immutables-macro README](https://github.com/defi-wonderland/aztec-immutables-macro/tree/dev).