extract: join a usage line's bracket group only when it is flat

[sadigaxund]

S-154's multi-word join tested no flatness, so it also fused a nested bracket group, and uniq [OPTION]... [INPUT [OUTPUT]] reached the tree as one positional called INPUT OUTPUT; the join now requires one bracket pair. The sweep that admitted the rule could not see it, because sweep-diff gates on flag and subcommand counts and a positional rename moves neither.

Positionals on a full-PATH sweep of 2324 tools. Every tool that carries a space-bearing positional on main is listed; nothing else in the sweep moves.

tool	group	main (0047400)	this branch	pre-merge (0b30c15)
accessdb	[MAN DATABASE]	MAN DATABASE	MAN DATABASE	DATABASE, MAN
gdk-pixbuf-thumbnailer	[INPUT FILE] [OUTPUT FILE]	INPUT FILE, OUTPUT FILE	INPUT FILE, OUTPUT FILE	FILE, INPUT, OUTPUT
mknod	[MAJOR MINOR]	NAME, TYPE, MAJOR MINOR	NAME, TYPE, MAJOR MINOR	MAJOR, MINOR, NAME, TYPE
systemd-sysusers	[CONFIGURATION FILE...]	CONFIGURATION FILE	CONFIGURATION FILE	CONFIGURATION, FILE
systemd-tmpfiles	[CONFIGURATION FILE...]	CONFIGURATION FILE	CONFIGURATION FILE	CONFIGURATION, FILE
chroot	[COMMAND [ARG]...]	NEWROOT, COMMAND ARG	NEWROOT, COMMAND, ARG...	ARG, COMMAND, NEWROOT
env	[COMMAND [ARG]...]	COMMAND ARG	COMMAND, ARG...	ARG, COMMAND
gettext	[[TEXTDOMAIN] MSGID]	TEXTDOMAIN MSGID	TEXTDOMAIN, MSGID	MSGID, TEXTDOMAIN
grub-menulst2cfg	[INFILE [OUTFILE]]	INFILE OUTFILE	INFILE, OUTFILE	INFILE, OUTFILE
inetutils-telnet	[HOST [PORT]]	HOST PORT	HOST, PORT	HOST, PORT
mkpasswd	[PASSWORD [SALT]]	PASSWORD SALT	PASSWORD, SALT	PASSWORD, SALT
nice	[COMMAND [ARG]...]	COMMAND ARG	COMMAND, ARG...	ARG, COMMAND
parted	[DEVICE [COMMAND [PARAMETERS]...]...]	DEVICE COMMAND PARAMETERS	DEVICE, COMMAND, PARAMETERS...	COMMAND, DEVICE, PARAMETERS
patch	[ORIGFILE [PATCHFILE]]	ORIGFILE PATCHFILE	ORIGFILE, PATCHFILE	ORIGFILE, PATCHFILE
perror	[ERRORCODE [ERRORCODE...]]	ERRORCODE ERRORCODE	ERRORCODE	ERRORCODE
ptx	[INPUT [OUTPUT]]	INPUT..., INPUT OUTPUT	INPUT..., OUTPUT	INPUT, OUTPUT
split	[FILE [PREFIX]]	FILE PREFIX	FILE, PREFIX	FILE, PREFIX
sqlite3	[FILENAME [SQL]]	FILENAME SQL	FILENAME, SQL	FILENAME, SQL
telnet	[HOST [PORT]]	HOST PORT	HOST, PORT	HOST, PORT
uniq	[INPUT [OUTPUT]]	INPUT OUTPUT	INPUT, OUTPUT	INPUT, OUTPUT

The first five groups are flat and stay joined. The other fifteen are nested and read exactly as they did before the rule shipped: for fourteen of them the whole #fp2 record is byte-identical to its pre-merge record, and sqlite3's remaining difference is nine value names S-157 gave it.

Sweep-diff against the merged sweep: 0 flag losses, 0 flag gains, 0 subcommand movement, 0 status transitions, 0 tools appeared or disappeared, field-level changes on exactly those 15 tools. All nine named controls byte-identical. The existence oracle reads 81 tools against 95. nested-bracket-group-fused-operand reads 15 tools/15 findings on the fused parser and 0/0 on this branch, ratcheted at zero.