keydb: round-trip AACS 2.0 host certs; tighten entry validation

- to_keydb_cfg emits the sibling `| HC2 |` line (AACS 2.0 host cert),
  the inverse of parse_host_cert_v2, so a round-trip never silently
  drops v2 host certs.
- save-validation mirrors the parser's real disc-entry rule exactly: a
  `0x` line counts as an entry only if it also contains ` = `, so
  validating + persisting content that parses to zero usable entries
  (e.g. a stray `0xDEADBEEF` line) can no longer succeed.
- disc-entry title is kept verbatim (parens and all) on parse, matching
  the emit path so it round-trips.

src/keydb.rs

@@ -90,7 +90,11 @@ impl KeydbSource {
             .lines()
             .filter(|l| {
                 let t = l.trim();
-                t.starts_with("0x")
+                // Mirror KeyDb::parse's disc-entry rule EXACTLY (keydb_format.rs:
+                // a "0x" line is only an entry if it also contains " = "), so
+                // save() never validates + persists content that parses to zero
+                // usable entries (e.g. a stray "0xDEADBEEF" comment line).
+                (t.starts_with("0x") && t.contains(" = "))
                     || t.starts_with("| DK")
                     || t.starts_with("| PK")
                     || t.starts_with("| HC")
@@ -814,7 +818,7 @@ mod tests {
         let target = dir.join("nested").join("mykeys.cfg");
         let src = KeydbSource::new(&target);
 
-        let body = b"0xDEADBEEFDEADBEEFDEADBEEFDEADBEEF\n";
+        let body = b"0xDEADBEEFDEADBEEFDEADBEEFDEADBEEF = Test\n";
         let result = src.save(body).expect("save must succeed");
 
         assert_eq!(
@@ -840,7 +844,7 @@ mod tests {
         let target = dir.join("k.cfg");
         let src = KeydbSource::new(&target);
 
-        let body = b"0xAABBCCDDAABBCCDDAABBCCDDAABBCCDD\n".to_vec();
+        let body = b"0xAABBCCDDAABBCCDDAABBCCDDAABBCCDD = Test\n".to_vec();
         let result = src
             .update(|_url| Ok(body.clone()), "http://example.invalid/keydb.zip")
             .expect("update must succeed with a good fetch");

src/keydb_format.rs

@@ -390,6 +390,8 @@ impl KeyDb {
         let mut out = String::new();
 
         // Host certs (AACS 1.0): | HC | HOST_PRIV_KEY 0x.. | HOST_CERT 0x.. ; Revoked in MKBv<N>
+        // AACS 2.0 credentials ride a sibling `| HC2 |` line; emit it too so a
+        // round-trip through `to_keydb_cfg` never silently drops v2 host certs.
         for hc in &self.host_certs {
             out.push_str("| HC | HOST_PRIV_KEY 0x");
             out.push_str(&hx(&hc.cert.private_key));
@@ -400,6 +402,17 @@ impl KeyDb {
                 out.push_str(&n.to_string());
             }
             out.push('\n');
+            // AACS 2.0 (HC2): inverse of `parse_host_cert_v2`.
+            if let (Some(pk2), Some(cert2)) = (
+                hc.cert.private_key_v2.as_ref(),
+                hc.cert.certificate_v2.as_ref(),
+            ) {
+                out.push_str("| HC2 | HOST_PRIV_KEY 0x");
+                out.push_str(&hx(pk2));
+                out.push_str(" | HOST_CERT 0x");
+                out.push_str(&hx(cert2));
+                out.push('\n');
+            }
         }
 
         // Device keys: | DK | DEVICE_KEY 0x.. | DEVICE_NODE 0x.. | KEY_UV 0x.. | KEY_U_MASK_SHIFT 0x..
@@ -429,9 +442,8 @@ impl KeyDb {
             // `0x` would double it on re-parse).
             out.push_str(h);
             out.push_str(" = ");
-            // Parse stores the display title (inside parens) or the whole string
-            // when there are none; emitting the stored title bare round-trips
-            // (no parens → parser keeps it verbatim). Empty → "Unknown".
+            // Parse stores the title VERBATIM (parens and all), so emitting it
+            // bare round-trips through parse. Empty → "Unknown".
             if d.title.is_empty() {
                 out.push_str("Unknown");
             } else {
@@ -1096,8 +1108,8 @@ mod tests {
 
     #[test]
     fn disc_entry_malformed_parens_falls_back_to_whole_title() {
-        // ')' before '(' would make start+1 > end; the guarded get() returns
-        // None and the parser falls back to the whole title (no panic).
+        // The title is kept verbatim regardless of paren placement — a malformed
+        // ')' before '(' is not special-cased; the whole string is the title.
         let line = "0x00 = FILM) (X | M | 0x".to_string() + &"00".repeat(16);
         let e = KeyDb::parse_disc_entry(&line).unwrap();
         assert_eq!(e.title, "FILM) (X");