perf: share links use scrypt 2^15, about 40 ms each way

Keys keeping to share.keyRules are random and long, so age's 2^18
(a third of a second to share and to open) bought little.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

internal/share/share.go

@@ -51,10 +51,12 @@ const (
 	maxFiles = 1 << 20
 )
 
-// workFactor is age's scrypt work factor for sealing (its default);
-// maxWorkFactor is the most opening accepts, so a link can't make it
-// work for minutes.
-var workFactor, maxWorkFactor = 18, 20
+// workFactor is the scrypt work factor for sealing: 2^15, about 40 ms
+// each way, instead of age's 2^18 (a third of a second). scrypt makes
+// guessing a weak passphrase slow; a key keeping to the rules is random
+// and long enough not to need that. maxWorkFactor is the most opening
+// accepts, so a link can't make it work for minutes.
+var workFactor, maxWorkFactor = 15, 20
 
 // File is a shared file.
 type File struct {