Store mtime to the nanosecond so a same-second rewrite is re-hashed (closes #12)

scan recorded mtime in whole seconds, so a file rewritten in place at
the same size within the same second as its recorded mtime was classed
unchanged and kept its old hashes. The files table keeps mtime as whole
Unix seconds and gains mtime_nsec, the nanoseconds within that second.
scan holds the mtime as a time.Time and decides "newer" with After, so
any time a filesystem can record, one after 2262 included, compares in
the right order. The walk, a file given as an operand, and the content
phase's recheck all move over. PRAGMA user_version stays 1, per the
owner's ruling. README states what both columns hold.

Model: opus-5-5

scan.go

@@ -17,6 +17,7 @@ import (
 	"strings"
 	"sync"
 	"syscall"
+	"time"
 )
 
 // The duplicate ladder (see hashSignature and README "Duplicate
@@ -68,7 +69,7 @@ var errInterrupted = errors.New("scan interrupted")
 type fileRec struct {
 	path  string
 	size  int64
-	mtime int64
+	mtime time.Time
 	dev   uint64
 	ino   uint64
 }
@@ -79,7 +80,7 @@ type fileRec struct {
 // they would dominate the scan's memory.
 type fileMeta struct {
 	size   int64
-	mtime  int64
+	mtime  time.Time
 	hashed bool
 }
 
@@ -369,7 +370,7 @@ func (s *scanState) loadIndex(ctx context.Context, roots []string) error {
 	s.existing = make(map[string]fileMeta)
 
 	return loadFileMeta(ctx, s.db,
-		func(path string, size, mtime int64, hashed bool) {
+		func(path string, size int64, mtime time.Time, hashed bool) {
 			prog.increment()
 
 			if underAnyRoot(path, roots) {
@@ -415,7 +416,7 @@ func (s *scanState) walkPhase(
 		old, ok := s.existing[ev.rec.path]
 
 		switch {
-		case !ok || old.size != ev.rec.size || old.mtime < ev.rec.mtime:
+		case !ok || old.size != ev.rec.size || ev.rec.mtime.After(old.mtime):
 			changed = append(changed, ev.rec)
 		case old.hashed:
 			delete(s.existing, ev.rec.path)
@@ -807,7 +808,7 @@ func unchangedFile(r scanRec) (fileRec, bool, error) {
 	}
 
 	if !fi.Mode().IsRegular() || fi.Size() != r.size ||
-		fi.ModTime().Unix() > r.mtime {
+		fi.ModTime().After(r.mtime) {
 		return fileRec{}, false, nil
 	}
 
@@ -966,7 +967,7 @@ func seedRoot(ctx context.Context, root string,
 	sendEvent(ctx, events, walkEvent{rec: fileRec{
 		path:  root,
 		size:  fi.Size(),
-		mtime: fi.ModTime().Unix(),
+		mtime: fi.ModTime(),
 		dev:   dev,
 		ino:   ino,
 	}})
@@ -1124,7 +1125,7 @@ func emitFile(ctx context.Context, p string, e fs.DirEntry,
 	sendEvent(ctx, events, walkEvent{rec: fileRec{
 		path:  p,
 		size:  info.Size(),
-		mtime: info.ModTime().Unix(),
+		mtime: info.ModTime(),
 		dev:   dev,
 		ino:   ino,
 	}})