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" by comparing
Unix() and then Nanosecond(), so any time a filesystem can record
compares in the right order; After would misorder one too late for a
time.Time to hold without wrapping. 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

db_test.go

@@ -10,6 +10,7 @@ import (
 	"slices"
 	"strings"
 	"testing"
+	"time"
 )
 
 // testDBPath returns a database path inside a fresh temp dir.
@@ -260,10 +261,13 @@ func TestApplyChangesRoundTrip(t *testing.T) {
 	// written.
 	recs := []scanRec{
 		{
-			size: 2, mtime: 20, head: "h2", tail: "t2", content: "c2",
-			path: "/a/tab\tnew\nline",
+			size: 2, mtime: time.Unix(20, 999_999_999), head: "h2", tail: "t2",
+			content: "c2", path: "/a/tab\tnew\nline",
+		},
+		{
+			size: 1, mtime: time.Unix(10, 0), head: "h1", tail: "t1",
+			content: "c1", path: "/a/x",
 		},
-		{size: 1, mtime: 10, head: "h1", tail: "t1", content: "c1", path: "/a/x"},
 	}
 
 	err := applyChanges(t.Context(), db, recs, nil,
@@ -281,7 +285,8 @@ func TestApplyChangesRoundTrip(t *testing.T) {
 	// An upsert for an existing path updates in place; a delete
 	// removes exactly its path.
 	upd := scanRec{
-		size: 3, mtime: 30, head: "h3", tail: "t3", content: "c3", path: "/a/x",
+		size: 3, mtime: time.Unix(30, 0), head: "h3", tail: "t3", content: "c3",
+		path: "/a/x",
 	}
 
 	err = applyChanges(t.Context(), db, []scanRec{upd},
@@ -308,7 +313,7 @@ func TestApplyChangesBatching(t *testing.T) {
 	recs := make([]scanRec, 0, n)
 	for i := range n {
 		recs = append(recs, scanRec{
-			size: int64(i), mtime: 1, head: "h", tail: "t",
+			size: int64(i), mtime: time.Unix(1, 0), head: "h", tail: "t",
 			path: fmt.Sprintf("/batch/%07d", i),
 		})
 	}