Extract validation helpers to a dedicated file, add static file bundling
for base templates, and add Windows OS support.
**Commit 1: Extract validation to `validate.go`**
Move `validateVariableValue`, `validateStringValue`,
`validateNumberValue`, `validateBoolValue`, `toHCLLiteral`, `hclQuote`,
and `isSimpleJSONValue` from `compose.go` into `validate.go`.
Corresponding tests move to `validate_internal_test.go`. This keeps
`compose.go` focused on the compose pipeline.
**Commit 2: Static file bundling**
Add `StaticFiles` field to `ComposeResult` and a `collectStaticFiles`
helper that walks the base template FS to collect non-template files
(e.g. cloud-init `.tftpl` inputs). `BundleTar` now writes these files
into the output archive in sorted order for deterministic output. This
fixes `aws-linux`, whose cloud-init files were embedded but never
included in the tar.
**Commit 3: Windows OS support**
Add `BaseOSWindows` constant and register `"windows"` in `validBaseOS`
so that base templates with `os="windows"` can be loaded and used for
module compatibility filtering.
> [!NOTE]
> This PR was authored by Coder Agents on behalf of @jeremyruppel.
