docs: Documenting mutable attribute

Author: yhakbar

docs/src/content/docs/03-features/02-stacks/03-explicit.mdx

@@ -412,7 +412,19 @@ When `update_source_with_cas = true` is set:
 
 Consumers do not set `update_source_with_cas` themselves. When the `cas` experiment is enabled and the source is remote, Terragrunt uses the CAS path automatically. The attribute only has effect inside catalog files, where it flags nested `source` attributes for rewriting.
 
-`unit` and `stack` blocks also accept a `mutable` attribute. When `mutable = true`, the unit's or stack's content under `.terragrunt-stack` is copied from the CAS store instead of hardlinked. The default is `false`, which lets CAS hardlink files for speed and deduplication. Set this when you intend to edit files in `.terragrunt-stack` directly: hardlinked files share an inode with the shared store, so editing them in place would corrupt it.
+<Before version="1.0.4">
+
+<Aside type="tip">
+A `mutable` attribute that opts a `unit` or `stack` block out of CAS hardlinking is coming in v1.0.4.
+</Aside>
+
+</Before>
+
+<Since version="1.0.4">
+
+`unit` and `stack` blocks also accept a `mutable` attribute. When `mutable = true`, the unit's or stack's content under `.terragrunt-stack` is copied from the CAS store instead of hardlinked. The default is `false`, which lets CAS hardlink files for speed and deduplication. Hardlinked files inherit the store's read-only permissions, so the working tree is safe by default; set `mutable = true` when you want to edit files in `.terragrunt-stack` directly.
+
+</Since>
 
 Given the catalog files above, and an ordinary consumer stack like:
 

docs/src/content/docs/04-reference/01-hcl/02-blocks.mdx

@@ -8,6 +8,8 @@ sidebar:
 
 import FileTree from '@components/vendored/starlight/FileTree.astro';
 import { Aside } from '@astrojs/starlight/components';
+import Since from '@components/Since.astro';
+import Before from '@components/Before.astro';
 
 Terragrunt HCL configuration uses [configuration blocks](https://github.com/hashicorp/hcl/blob/main/hclsyntax/spec.md#blocks) when there's a structural configuration that needs to be defined for Terragrunt.
 
@@ -69,13 +71,6 @@ The `terraform` block supports the following arguments:
 
   *Note that if you wish to exclude files from being copied from a terraform module source, you should use the [before_hook](/features/units/hooks) feature.*
 
-- `mutable` (attribute): When `true`, content fetched into `.terragrunt-cache` is copied from the
-  [content-addressable storage (CAS)](/features/caching/cas) instead of hardlinked. The default is `false`,
-  which lets CAS hardlink files from its shared store for speed and deduplication. Set this to `true` when you
-  intend to edit files inside `.terragrunt-cache` directly: hardlinked files share an inode with the CAS store,
-  so editing them in place would corrupt the store. The flag has no effect when CAS is not used to fetch the
-  source; the standard download path already produces an independent copy.
-
 - `copy_terraform_lock_file` (attribute): In certain use cases, you don't want to check the terraform provider lock
   file into your source repository from your working directory as described in
   [Lock File Handling](/reference/lock-files). This attribute allows you to disable the copy
@@ -118,6 +113,26 @@ The `terraform` block supports the following arguments:
   error must match one of the expressions listed in the `on_errors` attribute. Error hooks are executed after the before/after hooks.
   To handle errors during source download (when using the `source` attribute), use `init-from-module` in the `commands` list.
 
+<Before version="1.0.4">
+
+<Aside type="tip">
+A `mutable` attribute that opts a `terraform` block out of CAS hardlinking is coming in v1.0.4.
+</Aside>
+
+</Before>
+
+<Since version="1.0.4">
+
+- `mutable` (attribute): When `true`, content fetched into `.terragrunt-cache` through the
+  [content-addressable storage (CAS)](/features/caching/cas) is copied from the CAS store instead of
+  hardlinked. The default is `false`, which lets CAS hardlink files from its shared store for speed and
+  deduplication. Hardlinked files inherit the store's read-only permissions, so the working tree is safe
+  by default; set `mutable = true` when you want to edit files inside `.terragrunt-cache` directly. The
+  flag has no effect when CAS is not used to fetch the source; the standard download path already
+  produces an independent, writable copy.
+
+</Since>
+
 In addition to supporting before and after hooks for all OpenTofu/Terraform commands, the following specialized hooks are also
 supported:
 

docs/src/data/changelog/v1.0.4/cas-mutable-sources.mdx

@@ -0,0 +1,19 @@
+---
+version: "v1.0.4"
+category: "experiments-updated"
+---
+
+#### `cas` — `mutable` attribute on `terraform`, `unit`, and `stack` blocks
+
+A new `mutable` attribute opts a block out of CAS hardlinking when its source is fetched through CAS. With `mutable = true`, files materialized into `.terragrunt-cache` (for `terraform`) or `.terragrunt-stack` (for `unit` and `stack`) are copied from the CAS store rather than hardlinked from it.
+
+The default is `false`, which preserves the existing hardlink behavior. Hardlinked files inherit the CAS store's read-only permissions, so the working tree is safe by default; set `mutable = true` when you want to edit files inside the destination directory.
+
+```hcl
+terraform {
+  source  = "git::https://github.com/acme/infrastructure-modules.git//vpc?ref=v1.0.0"
+  mutable = true
+}
+```
+
+The flag is orthogonal to `update_source_with_cas` and has no effect when content is fetched through the standard download path, which already produces an independent copy.