Keep the empty host for a state_dir starting with //

With OmitHost set, a state_dir starting with // gave a db_url starting
file://, and SQLite read the first directory name as a host, so pixa
failed to start. OmitHost is now left off for such a directory: the URL
starts file://// and SQLite reads an empty host followed by the path.
Every other state_dir gives the same URL as before.

Model: opus-5-5

README.md

@@ -726,12 +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`. 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`
+  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
 - `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