vmm: add per-zone mergeable option to --memory-zone

Add a `mergeable` field to `MemoryZoneConfig` so that KSM page merging
can be enabled selectively per memory zone rather than globally for all
guest RAM.

Previously, `MADV_MERGEABLE` was only controllable via the top-level
`--memory mergeable=on` flag, which applied uniformly to all regions.
With this change, users can leave boot memory unmerged while enabling
KSM only on hotplug zones:

  --memory size=0,hotplug_method=virtio-mem
  --memory-zone id=boot,size=512M,shared=on,mergeable=off
  --memory-zone id=hotplug,size=256M,hotplug_size=1G,shared=off,mergeable=on

The `MemoryZone` runtime struct now carries the `mergeable` flag so
that both `allocate_address_space` and `add_ram_region` can apply
per-zone `MADV_MERGEABLE` instead of the global `self.mergeable`.
The top-level `--memory mergeable=on` path continues to work unchanged:
the default zone is synthesised from `MemoryConfig` and inherits its
`mergeable` value.

AI/LLM disclosure: this patch was co-authored with
GitHub Copilot and Claude Code (Opus 4.6).

Signed-off-by: JP Kobryn <inwardvessel@gmail.com>

docs/memory.md

@@ -214,11 +214,12 @@ struct MemoryZoneConfig {
     hotplug_size: Option<u64>,
     hotplugged_size: Option<u64>,
     prefault: bool,
+    mergeable: bool,
 }
 ```
 
 ```
---memory-zone <memory-zone>	User defined memory zone parameters "size=<guest_memory_region_size>,file=<backing_file>,shared=on|off,hugepages=on|off,hugepage_size=<hugepage_size>,host_numa_node=<node_id>,id=<zone_identifier>,hotplug_size=<hotpluggable_memory_size>,hotplugged_size=<hotplugged_memory_size>,prefault=on|off"
+--memory-zone <memory-zone>	User defined memory zone parameters "size=<guest_memory_region_size>,file=<backing_file>,shared=on|off,hugepages=on|off,hugepage_size=<hugepage_size>,host_numa_node=<node_id>,id=<zone_identifier>,hotplug_size=<hotpluggable_memory_size>,hotplugged_size=<hotplugged_memory_size>,prefault=on|off,mergeable=on|off"
 ```
 
 This parameter expects one or more occurrences, allowing for a list of memory
@@ -422,6 +423,34 @@ _Example_
 --memory-zone id=mem0,size=1G,prefault=on
 ```
 
+### `mergeable`
+
+Specifies if the pages from this memory zone must be marked as _mergeable_,
+enabling Kernel Same-page Merging (KSM) for this zone.
+
+This is the per-zone equivalent of the top-level `--memory mergeable=on` option.
+It allows KSM to be enabled selectively — for example, enabling it only on a
+hotplug zone while leaving boot memory unaffected:
+
+```
+--memory size=2G,mergeable=off
+--memory-zone id=hotplug,size=0,hotplug_size=8G,mergeable=on
+```
+
+For KSM to have any effect, the host kernel must have KSM enabled:
+```
+echo 1 > /sys/kernel/mm/ksm/run
+```
+
+By default this option is turned off.
+
+_Example_
+
+```
+--memory size=0
+--memory-zone id=mem0,size=1G,mergeable=on
+```
+
 ## NUMA settings
 
 `NumaConfig` or what is known as `--numa` from the CLI perspective has been