all: cut bloat found in the pike audit

Dead, vestigial, and over-built code with no behavior change (except the
bool-vocabulary trim, a deliberate behavior tightening):

- httpdl: drop the vestigial sort+copy in validSegs (work-stealing was
  removed in 270812d, so segments always tile [0,total) ascending now);
  validate order in place, which also rejects a scrambled sidecar. Inline
  the endOff() accessor to s.end.
- cli: shrink the boolean vocabulary to true/false, which is all that's
  accepted. The invented yes/no/1/0/on/off spellings are gone, so e.g.
  --enable-dht=yes (or yes in got.conf) is now a usage error.
- download: remove the dead, never-called Stat.Done() method.
- progress: drop speed(), a pure alias of humanSize; call humanSize directly.
- main: replace the jobsHolder mutex box with atomic.Pointer[[]job] for the
  forced-exit session save; the capability stays, the lock and type go.
- bt: collapse isAddrInUse's X||X (missinggo.IsAddrInUse is that exact
  string match); drop the now-unused missinggo import (tidy -> indirect).
- option: note that prealloc and falloc are identical here.

main.go

@@ -17,7 +17,7 @@ import (
 	"sort"
 	"strconv"
 	"strings"
-	"sync"
+	"sync/atomic"
 	"syscall"
 	"time"
 
@@ -94,8 +94,12 @@ func run(args []string) int {
 		t := time.AfterFunc(time.Duration(stop)*time.Second, cancel)
 		defer t.Stop()
 	}
-	jobsRef := &jobsHolder{}
-	go handleSignals(cancel, sessionFile, jobsRef)
+	var jobsRef atomic.Pointer[[]job]
+	// publish snapshots the slice header (j is a by-value copy) and stores a
+	// pointer to it, so the signal handler can read the current jobs lock-free
+	// without racing the `jobs = append(...)` reassignment in the follow pass.
+	publish := func(j []job) { jobsRef.Store(&j) }
+	go handleSignals(cancel, sessionFile, &jobsRef)
 
 	eng := download.NewEngine(opts.Int("max-concurrent-downloads"))
 
@@ -111,7 +115,7 @@ func run(args []string) int {
 		close(uiDone)
 	}
 
-	jobsRef.set(jobs)
+	publish(jobs)
 	results := runJobs(ctx, eng, jobs)
 
 	// --follow-torrent: a .torrent fetched over HTTP becomes a BitTorrent
@@ -125,7 +129,7 @@ func run(args []string) int {
 			// Index space, so the sort below restores input order deterministically.
 			nInitial := len(jobs)
 			jobs = append(jobs, follow...)
-			jobsRef.set(jobs)
+			publish(jobs)
 			followResults := runJobs(ctx, eng, follow)
 			for i := range followResults {
 				followResults[i].Index += nInitial
@@ -153,25 +157,6 @@ func run(args []string) int {
 	return report(results)
 }
 
-// jobsHolder lets the signal handler reach the latest job list for a
-// best-effort session save on a forced (second-Ctrl-C) exit.
-type jobsHolder struct {
-	mu   sync.Mutex
-	jobs []job
-}
-
-func (h *jobsHolder) set(jobs []job) {
-	h.mu.Lock()
-	h.jobs = jobs
-	h.mu.Unlock()
-}
-
-func (h *jobsHolder) get() []job {
-	h.mu.Lock()
-	defer h.mu.Unlock()
-	return h.jobs
-}
-
 // job pairs a download with the source string that produced it, so we can write
 // the still-unfinished ones back to a session file.
 type job struct {
@@ -810,14 +795,18 @@ func progressNote(s download.Stat) string {
 // TERM) cancels the graceful context so run() can wind down and save normally;
 // a second forces exit, but first writes the session best-effort so the resume
 // list survives a force-quit.
-func handleSignals(cancel context.CancelFunc, sessionFile string, jobs *jobsHolder) {
+func handleSignals(cancel context.CancelFunc, sessionFile string, jobs *atomic.Pointer[[]job]) {
 	sig := make(chan os.Signal, 2)
 	signal.Notify(sig, os.Interrupt, syscall.SIGTERM)
 	<-sig // first: cancel the graceful context
 	cancel()
 	<-sig // second: force exit
 	if sessionFile != "" {
-		if err := saveSession(sessionFile, jobs.get()); err != nil {
+		var js []job // nil until the first publish; saveSession handles an empty list
+		if p := jobs.Load(); p != nil {
+			js = *p
+		}
+		if err := saveSession(sessionFile, js); err != nil {
 			fmt.Fprintln(os.Stderr, "got:", err)
 		}
 	}