refactor(browser): split installation and profile abstractions

A Chromium installation shares one master key across its profiles, but
modeling each profile as its own Browser re-derived the key per profile.
Browser now represents one installation holding its profiles and derives
the key once; new types.Profile/ExtractResult/CountResult carry per-profile
results.

types/result.go

@@ -0,0 +1,19 @@
+package types
+
+// Profile identifies one browser profile — a leaf under an installation.
+type Profile struct {
+	Name string
+	Dir  string
+}
+
+// ExtractResult pairs a profile with the data extracted from it.
+type ExtractResult struct {
+	Profile
+	Data *BrowserData
+}
+
+// CountResult pairs a profile with its per-category entry counts.
+type CountResult struct {
+	Profile
+	Counts map[Category]int
+}