refactor: convert user_output.rs to folder module structure

- Created user_output/ folder with organized submodules
- Separated core components: theme, channel, verbosity, traits
- Organized messages into messages/ subdirectory (7 message types)
- Organized sinks into sinks/ subdirectory (4 sink implementations + writers)
- Organized formatters into formatters/ subdirectory (JsonFormatter)
- Extracted test_support module for test utilities
- Co-located all 155 tests with code in core.rs
- Maintained 100% backward compatibility - all public APIs unchanged
- All tests passing

Co-authored-by: josecelano <[email protected]>

Author: Copilot

src/presentation/user_output/channel.rs

@@ -0,0 +1,27 @@
+//! Output channel routing for user-facing messages
+//!
+//! This module defines the channel enum that determines whether messages
+//! should be written to stdout or stderr.
+
+/// Output channel for routing messages
+///
+/// Determines whether a message should be written to stdout or stderr.
+/// Following Unix conventions:
+/// - **stdout**: Final results and structured data for piping/redirection
+/// - **stderr**: Progress updates, status messages, operational info, errors
+///
+/// # Examples
+///
+/// ```rust
+/// use torrust_tracker_deployer_lib::presentation::user_output::Channel;
+///
+/// let channel = Channel::Stdout;
+/// assert_eq!(channel, Channel::Stdout);
+/// ```
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Channel {
+    /// Standard output stream for final results and data
+    Stdout,
+    /// Standard error stream for progress and operational messages
+    Stderr,
+}