feat(focas): coalesce adjacent PMC reads into one pmc_rdpmcrng (#266)

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Contracts/FocasDriverOptions.cs

@@ -31,6 +31,8 @@ public sealed class FocasDriverOptions
     public FocasHandleRecycleOptions HandleRecycle { get; init; } = new();
     /// <summary>Gets the fixed tree options.</summary>
     public FocasFixedTreeOptions FixedTree { get; init; } = new();
+    /// <summary>Gets the PMC range-coalescing options (Gitea #266).</summary>
+    public FocasPmcCoalesceOptions PmcCoalesce { get; init; } = new();
 
     /// <summary>
     ///     Timeout for the AdminUI Test Connect probe, in seconds. The AdminUI clamps to a
@@ -41,6 +43,32 @@ public sealed class FocasDriverOptions
     public int ProbeTimeoutSeconds { get; init; } = 10;
 }
 
+/// <summary>
+///     Merges adjacent / near PMC reads in one poll batch into fewer <c>pmc_rdpmcrng</c> calls (Gitea #266).
+///     On by default: a batch with a single PMC tag per area is unaffected, and a coalesced read the CNC
+///     rejects falls back to per-tag reads, so enabling it cannot make a readable tag unreadable.
+/// </summary>
+public sealed class FocasPmcCoalesceOptions
+{
+    /// <summary>Gets a value indicating whether PMC coalescing is on.</summary>
+    public bool Enabled { get; init; } = true;
+
+    /// <summary>
+    ///     Largest range, in bytes, one coalesced read may cover. Bounds the response size and the
+    ///     blast radius of a rejected range. Default 128.
+    /// </summary>
+    [Range(1, 1024)]
+    public int MaxSpanBytes { get; init; } = 128;
+
+    /// <summary>
+    ///     Largest run of bytes nobody asked for that may sit between two merged tags. Reading a few
+    ///     spare bytes is cheaper than a second round-trip, but every spare byte must be a valid PMC
+    ///     address on the CNC. Default 8; 0 merges only strictly adjacent tags.
+    /// </summary>
+    [Range(0, 1024)]
+    public int MaxGapBytes { get; init; } = 8;
+}
+
 /// <summary>
 ///     Fixed-node tree exposed by FOCAS per <c>docs/v2/driver-specs.md §7</c> —
 ///     <c>Identity/</c>, <c>Axes/{name}/</c>, etc. populated from