filesys: a handle on an existing file is writable

[codewiz]

Workbench could not snapshot icons on a HOSTFS volume: the Snapshot ended
in a seek error.

The packet trace shows icon.library's path -- lock the .info, OpenFromLock
it, read the header, write it back through the same handle:

locate "Boing.info" (lock 0x3AA0A8)
packet type 8    -> res1=0x3AA1E0 res2=0      ACTION_LOCATE_OBJECT
packet type 1026 -> res1=0xFFFFFFFF res2=0    ACTION_FH_FROM_LOCK, ok
packet type 82   -> res1=0x3A res2=0          ACTION_READ, 58 bytes
packet type 87   -> res1=0xFFFFFFFF res2=219  ACTION_WRITE, seek error
packet type 1007 -> res1=0xFFFFFFFF res2=0    ACTION_END

ACTION_FH_FROM_LOCK opened the host file with File::open, i.e.
read-only, so write_all failed with a host error that host_error falls
through to ERROR_SEEK_ERROR. ACTION_FINDINPUT had the same bug:
Open(MODE_OLDFILE) yields a read/write handle on the Amiga, and
OpenFromLock's handle is writable regardless of the lock's access mode.

Both now go through open_existing(), which opens read/write and falls
back to read-only if the host refuses -- a later write then fails, which
is what a protected file does on the Amiga too.

Covered by a new test driving both open paths through a real write and
checking the bytes on disk.

🤖 Generated with Claude Code