httpdl: fix progress-reporter race on name/out; harden mirror & resume validation

Findings from a Rob-Pike-lens review (bugs/races/network), each verified
against the code before fixing:

- httpdl: name/out are now atomic.Pointer[string] -- the engine publishes a
  download to the reporter before Run resolves the name, so Stat raced the
  write (bt already did this)
- httpdl: a malformed --proxy fails loudly instead of silently bypassing it
- httpdl: a 206 must carry a matching Content-Range; a 200 in segmented mode is
  fatal so it fails over instead of burning the retry budget
- httpdl: single-stream mirror failover validates the range before appending;
  ErrTooSlow only when the error is a real ctx cancellation
- httpdl: idle guard tracks progress by timestamp (no Reset/Stop race, no
  sticky fired flag)
- httpdl/control: reject a resume file whose segments don't tile [0,total)
- bt: clamp the listen-port range; verify on-disk data before choosing pieces
  under --check-integrity
- cli: reject size overflow; show --seed-time=MIN; clamp --select-file range
- progress/engine/main: clamp ETA against int64 overflow; show queued
  downloads as waiting; join the reporter on exit instead of a 20ms sleep

httpdl/control.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"os"
 	"path/filepath"
+	"sort"
 	"sync/atomic"
 )
 
@@ -75,7 +76,35 @@ func loadControl(out, url string, total int64, etag, lastmod string) *control {
 	if (lastmod != "" || c.LastModified != "") && c.LastModified != lastmod {
 		return nil
 	}
+	// Reject a control file whose segments do not exactly tile [0,total): a
+	// truncated/corrupted/hand-edited sidecar that still parses as JSON could
+	// otherwise mark a segment done() without its bytes on disk (inflated Written)
+	// or leave an un-downloaded hole, both of which would be reported as a complete
+	// file. We restart cleanly instead of trusting it.
+	if !validSegs(c.Segs, c.Total) {
+		return nil
+	}
 	return &c
 }
 
+// validSegs reports whether segs cover [0,total) with no gap or overlap and a
+// sane written count for each. The recorded order is not sorted (a steal appends
+// a tail), so we check coverage on a sorted copy.
+func validSegs(segs []segState, total int64) bool {
+	if total <= 0 || len(segs) == 0 {
+		return false
+	}
+	sorted := append([]segState(nil), segs...)
+	sort.Slice(sorted, func(i, j int) bool { return sorted[i].Start < sorted[j].Start })
+	var next int64
+	for _, s := range sorted {
+		length := s.End - s.Start + 1
+		if s.Start != next || s.End < s.Start || s.Written < 0 || s.Written > length {
+			return false
+		}
+		next = s.End + 1
+	}
+	return next == total
+}
+
 func removeControl(out string) { os.Remove(controlPath(out)) }