Add live regression test for growing-file transfers, bump grace period to 1s

scripts/regression_test.sh exercises both writer behaviors -- normal
append-only growth (the original, must-keep-working case) and preallocated
growth (ftruncate-extend then shrink-and-rename, the case that caused the
hang/corruption fixed in the previous commit) -- in both transfer
directions, against a real ssh test host with a real capture file sliced to
a record-aligned prefix. scripts/pcap_offsets.py computes record-boundary
cut points for staging realistic growth.

Bump FINALIZE_GRACE_PERIOD from 500ms to 1s: a comfortable margin even for
a writer whose header and payload writes aren't atomic -- if the header
landed, the body should follow shortly.

src/tail.rs

@@ -18,8 +18,10 @@ const READ_WINDOW: usize = 1024 * 1024;
 /// visible via `fstat` slightly apart from each other -- NOT a guess about
 /// whether the writer finished; the authoritative "writer is done" signal
 /// is the rename itself (`complete_signal`). Re-checked continuously (reset
-/// on every observed length change), not a one-shot timer.
-const FINALIZE_GRACE_PERIOD: Duration = Duration::from_millis(500);
+/// on every observed length change), not a one-shot timer. 1s gives a
+/// comfortable margin even for a writer whose header and payload writes
+/// aren't atomic (if the header landed, the body should follow shortly).
+const FINALIZE_GRACE_PERIOD: Duration = Duration::from_secs(1);
 
 pub enum SourceEvent {
     Chunk(ChunkKind, Vec<u8>),