fix(io): handle file shrink in update_for_append to prevent SIGBUS

File truncation/rotation during mmap lifetime caused SIGBUS crash
because update_for_append() ignored new_size < old_size, leaving
a stale mapping that would fault on access.

Introduce AppendStatus enum (Unchanged/Appended/Reloaded) so the
caller can distinguish shrink events from normal appends. On shrink,
reload() rebuilds the mmap and line index. The TUI layer clamps
cursor and invalidates viewport cache on Reloaded, matching the
existing handle_file_truncated() behavior.

Fixes #1

crates/core/src/io/progressive_reader.rs

@@ -3,7 +3,7 @@ use std::fmt;
 use std::path::{Path, PathBuf};
 
 use crate::error::{CoreError, Result};
-use crate::io::file_reader::FileReader;
+use crate::io::file_reader::{AppendStatus, FileReader};
 use crate::io::index_cache::IndexCache;
 use crate::io::line_index::LineIndex;
 use crate::io::line_sampler::sample_line_count;
@@ -656,10 +656,10 @@ impl ProgressiveFileReader {
         }
     }
 
-    pub fn update_for_append(&mut self) -> Result<u64> {
+    pub fn update_for_append(&mut self) -> Result<AppendStatus> {
         match &mut self.state {
             ReaderState::Ready { reader, .. } => reader.update_for_append(),
-            _ => Ok(0),
+            _ => Ok(AppendStatus::Unchanged),
         }
     }