shell: make every -collection flag parse the same way (#11794)

* shell: make every -collection flag parse the same way

The filter vocabulary (comma-separated names, wildcards, regexes,
"_default" for the empty-named collection) already existed in
wildcard.CompileCollectionMatcher, but several commands still did their
own exact-match comparison or sent the raw flag value to servers.

fs.mergeVolumes used a literal string compare, so -collection=_default
silently produced an empty merge plan; it now uses the shared matcher
while keeping "*" as the match-all default. volume.fsck and volume.mark
filter through the matcher too, and volume.vacuum resolves the filter
client-side into per-collection and per-volume requests, since no
Collection value scopes the VacuumVolume RPC to the unnamed collection
alone.

Commands that put one concrete collection name on the wire normalize
"_default" to "" (volume.grow, fs.configure, mq.topic.compact,
collection.delete, ec.rebuild, ec.balance), and the tier -volumeId
commands resolve the volume's actual collection from topology first so
patterns and named-collection requests pass the volume server's
collection check.

* shell: spell the default-collection keyword via CollectionDefault

* shell: address review findings on collection filter handling

volume.fsck keeps a set of every volume the master knows before any
filtering, so a filer entry whose volume was excluded by -collection or
-volumeId is skipped instead of reported "volume not found" and deleted
by -reallyDeleteFilerEntries. The collection filter also no longer
scopes the filer walk to /buckets/<collection>: fs.configure can bind a
collection to any path prefix, and a narrower walk could mark live
chunks as orphans under -reallyDeleteFromVolume.

ec.rebuild and ec.balance resolve -collection through the new
wildcard.ExpandCollectionFilter, which passes literal names through even
when the master does not list them — ec.rebuild must be able to recover
a collection whose shards all lost their index and never registered.

fs.configure and mq.topic.compact now reject _default outright: an empty
Collection means "unset" in both the path-rule merge and the filer's
storage-option fallback chain, so the keyword would have been silently
ignored.

The unnamed-collection vacuum sweep keeps the master's low-disk-space
exception: a volume whose every replica is read-only is probed with
VacuumVolumeCheck and stays eligible when a replica reports
DiskSpaceLow, instead of being dropped outright.

weed/shell/command_ec_common_test.go

@@ -14,7 +14,7 @@ func TestCompileCollectionPattern(t *testing.T) {
 		{pattern: CollectionDefault, matches: []string{""}, misses: []string{"pictures"}},
 		{pattern: "pictures", matches: []string{"pictures"}, misses: []string{"", "pictures-backup"}},
 		{pattern: "pictures,videos", matches: []string{"pictures", "videos"}, misses: []string{"", "clips"}},
-		{pattern: "pictures,_default", matches: []string{"pictures", ""}, misses: []string{"clips"}},
+		{pattern: "pictures," + CollectionDefault, matches: []string{"pictures", ""}, misses: []string{"clips"}},
 		{pattern: "pictures*", matches: []string{"pictures", "pictures-backup"}, misses: []string{"clips"}},
 		{pattern: "^pictures", matches: []string{"pictures", "pictures-backup"}, misses: []string{"clips"}},
 		{pattern: "*", matches: []string{"", "pictures"}},