master: read the writable-volumes cap from master.toml (#12009)

* master: read the writable-volumes cap from master.toml

-maxWritableVolumes becomes the master.max_writable_volumes config key,
so the new master limit is set the same way as other master options
instead of a flag wired into every command that embeds a master.

* master: name the writable cap master.max_writable_volumes_per_layout

The cap applies per volume layout (collection + replica placement + ttl +
disk type), so the key names its scope instead of reading as a global cap.

* master: keep an embedder's MaxWritableVolumes when the key is unset

NewMasterServer overwrote option.MaxWritableVolumes with the viper
value, which reads as 0 whenever master.toml omits the key, silently
dropping a cap a Go caller supplied through MasterOption. Seed the
default from the option instead so the field survives an absent key;
an explicit config entry still wins.

* master: drop stray nested-checkout gitlinks

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

---------

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

weed/command/server.go

@@ -98,7 +98,6 @@ func init() {
 	masterOptions.volumeSizeLimitMB = cmdServer.Flag.Uint("master.volumeSizeLimitMB", util.DefaultVolumeSizeLimitMB, "Master stops directing writes to oversized volumes.")
 	masterOptions.volumePreallocate = cmdServer.Flag.Bool("master.volumePreallocate", false, "Preallocate disk space for volumes.")
 	masterOptions.maxParallelVacuumPerServer = cmdServer.Flag.Int("master.maxParallelVacuumPerServer", 1, "maximum number of volumes to vacuum in parallel on one volume server; 0 for no limit (deprecated, use master.vacuum.max_parallel_per_server in master.toml)")
-	masterOptions.maxWritableVolumes = cmdServer.Flag.Int("master.maxWritableVolumes", 0, "maximum writable volumes in a volume layout (collection + replica placement + ttl + disk type) assignments are spread over, fullest picked first; 0 means no limit")
 	masterOptions.vacuumIntervalSeconds = cmdServer.Flag.Int("master.vacuumIntervalSeconds", 840, "seconds between automatic vacuum sweeps")
 	masterOptions.vacuumDeleteEmptyAfterSeconds = cmdServer.Flag.Int("master.vacuumDeleteEmptyAfterSeconds", 0, "automatic sweep deletes volume copies that stay empty this many seconds; 0 disables")
 	masterOptions.defaultReplication = cmdServer.Flag.String("master.defaultReplication", "", "Default replication type if not specified.")