feat: add incremental log scanning with resumable byte offsets

Track last processed byte offset per log file so scans only read new
entries, add --full flag to force a rescan, and persist state atomically.

app/files.py

@@ -60,6 +60,26 @@ def read_lines(path: str) -> Iterator[str]:
             yield ANSI_RE.sub("", line.rstrip("\r\n"))
 
 
+def read_lines_from(path: str, start: int = 0) -> Iterator[tuple[str, int]]:
+    """Yield ``(line, resume_offset)`` for a plain-text log from byte ``start``.
+
+    Only seekable (non-gzip) files can be resumed. ``resume_offset`` points
+    after the last complete line; a trailing partial line (no newline yet) is
+    yielded but resets the resume offset to its start so it is re-read once
+    completed. The last yielded ``resume_offset`` is the value to store.
+    """
+    with open(path, "rb") as f:
+        f.seek(start)
+        pos = start
+        for raw in f:
+            line_start = pos
+            pos += len(raw)
+            complete = raw.endswith(b"\n") or raw.endswith(b"\r")
+            text = ANSI_RE.sub(
+                "", raw.decode("utf-8", errors="replace").rstrip("\r\n"))
+            yield text, (pos if complete else line_start)
+
+
 # -- writers ----------------------------------------------------------------
 
 def write_csv(path: str, header: Sequence[str],