Keep the empty host when the db_url path starts with //

defaultDBURL kept the empty host only when state_dir started with //,
but SQLite reads the path written into the URL, state_dir followed by
/state.sqlite3. For a state_dir of / that path is //state.sqlite3, the
URL started file:// and SQLite read state.sqlite3 as a host, so pixa
failed to start. The decision now looks at that path: a state_dir of
/ gives file:////state.sqlite3. Every other state_dir gives the same
URL as before.

Model: opus-5-5

README.md

@@ -726,13 +726,13 @@ Key settings in more detail:
 - `db_url` — the SQLite database to open; omitted, it is
   `file:<state_dir>/state.sqlite3?_pragma=journal_mode(WAL)`, which keeps the
   database in WAL mode. `state_dir` is percent-encoded in it, so a `%`, `?` or
-  `#` in the name is written as `%25`, `%3F` or `%23`. For a `state_dir`
-  starting with `//` it starts `file:////`, so SQLite does not read the first
-  directory name as a host. pixa opens one connection to it, so its own reads
-  and writes run one at a time. pixa adds `_pragma=busy_timeout(5000)` to any
-  `db_url`, so a write that finds another program writing to the file waits up
-  to five seconds for it instead of failing. WAL mode comes only from the URL:
-  keep `_pragma=journal_mode(WAL)` in one you set
+  `#` in the name is written as `%25`, `%3F` or `%23`. For a `state_dir` of `/`
+  or one starting with `//` it starts `file:////`, so SQLite does not read the
+  first name in the path as a host. pixa opens one connection to it, so its own
+  reads and writes run one at a time. pixa adds `_pragma=busy_timeout(5000)` to
+  any `db_url`, so a write that finds another program writing to the file waits
+  up to five seconds for it instead of failing. WAL mode comes only from the
+  URL: keep `_pragma=journal_mode(WAL)` in one you set
 - `cache_max_bytes` — disk cache size limit in bytes; `0` disables the disk
   cache entirely; omitted defaults to 75% of the sum of the free space on the
   filesystem containing `<state_dir>/cache/` and the bytes of source and