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

download/engine.go

@@ -46,6 +46,12 @@ func (e *Engine) Run(ctx context.Context, downloads []Download) []Result {
 		go func(i int, d Download) {
 			defer wg.Done()
 
+			// Track immediately so a download still queued for a concurrency slot
+			// is visible to the progress reporter as Waiting, rather than hidden
+			// until it starts running.
+			e.track(d)
+			defer e.untrack(d)
+
 			// Acquire a concurrency slot, or bail if we're shutting down
 			// before this download ever started.
 			select {
@@ -56,9 +62,6 @@ func (e *Engine) Run(ctx context.Context, downloads []Download) []Result {
 			}
 			defer func() { <-slots }()
 
-			e.track(d)
-			defer e.untrack(d)
-
 			err := d.Run(ctx)
 			results <- Result{Name: d.Name(), Index: i, Err: err, Stat: d.Stat()}
 		}(i, d)
@@ -74,7 +77,8 @@ func (e *Engine) Run(ctx context.Context, downloads []Download) []Result {
 	return out
 }
 
-// Snapshot returns a Stat for every currently running download.
+// Snapshot returns a Stat for every tracked download — those running plus those
+// still queued for a concurrency slot (reported as Waiting).
 func (e *Engine) Snapshot() []Stat {
 	e.mu.Lock()
 	defer e.mu.Unlock()