refactor(sql): promote the single-row limit onto ISqlDialect

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Sql/ISqlDialect.cs

@@ -6,9 +6,9 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Sql;
 
 /// <summary>
 ///     The provider seam (design §2.2). ADO.NET's <see cref="System.Data.Common"/> base types abstract
-///     connections, commands, parameters and readers; they do <b>not</b> abstract the two things that differ
-///     per backend — <b>identifier quoting</b> and the <b>metadata-catalog SQL</b>. Those live here, so no
-///     dialect assumption leaks into the reader, the poll planner, or the schema browser.
+///     connections, commands, parameters and readers; they do <b>not</b> abstract the three things that differ
+///     per backend — <b>identifier quoting</b>, the <b>metadata-catalog SQL</b>, and <b>row-limit syntax</b>.
+///     Those live here, so no dialect assumption leaks into the reader, the poll planner, or the schema browser.
 ///     <para><b>This interface is the driver's SQL-injection boundary.</b> Every runtime <em>value</em> is
 ///     bound as a <see cref="DbParameter"/> and never reaches a command text. Identifiers cannot be
 ///     parameterized in SQL, so the few that must appear as text are sourced only from catalog-validated
@@ -47,6 +47,41 @@ public interface ISqlDialect
     /// <summary>Cheapest statement that proves the connection is alive (<c>SELECT 1</c>; Oracle needs <c>FROM DUAL</c>).</summary>
     string LivenessSql { get; }
 
+    /// <summary>
+    ///     The fragment that goes <b>immediately after <c>SELECT</c></b> to limit a statement to one row —
+    ///     T-SQL's <c>"TOP 1 "</c>. Empty for a dialect that spells the limit at the end of the statement (see
+    ///     <see cref="SingleRowLimitSuffix"/>).
+    ///     <para><b>Include the trailing space</b> when non-empty: the planner concatenates
+    ///     <c>"SELECT " + SingleRowLimitPrefix + columns</c> with no separator of its own, so an empty fragment
+    ///     must leave the statement byte-identical to an unlimited one.</para>
+    /// </summary>
+    /// <remarks>
+    ///     <para><b>Why a prefix/suffix pair rather than one <c>ApplyRowLimit(sql, n)</c> method.</b> The two
+    ///     spellings sit at opposite ends of the statement — SQL Server writes
+    ///     <c>SELECT TOP 1 … ORDER BY … DESC</c>, while SQLite/Postgres/MySQL write
+    ///     <c>SELECT … ORDER BY … DESC LIMIT 1</c>. A single method taking the SELECT list could only serve the
+    ///     prefix position; a single method taking the whole statement would move statement assembly out of the
+    ///     planner and into every dialect. Two fragments keep the planner the sole author of statement shape
+    ///     and let each dialect fill in the end it uses.</para>
+    ///     <para><b>Why "single row" rather than a row count.</b> The only limit v1 emits is the wide-row
+    ///     <c>topByTimestamp</c> selector's newest-row pick. Modelling an arbitrary <c>n</c> would be untested
+    ///     surface, and <c>FETCH FIRST</c>/<c>ROWNUM</c> dialects need more than a substituted number anyway.
+    ///     Widen this to a method when a feature actually needs <c>n &gt; 1</c>.</para>
+    /// </remarks>
+    string SingleRowLimitPrefix { get; }
+
+    /// <summary>
+    ///     The fragment appended to the <b>very end</b> of a statement to limit it to one row —
+    ///     <c>" LIMIT 1"</c> for SQLite/Postgres/MySQL. Empty for a dialect that limits after <c>SELECT</c>
+    ///     (see <see cref="SingleRowLimitPrefix"/>).
+    ///     <para><b>Include the leading space</b> when non-empty, for the same reason the prefix carries a
+    ///     trailing one: the planner appends it directly to the finished statement.</para>
+    ///     <para>Exactly one of the two fragments is non-empty in every dialect modelled so far, but the planner
+    ///     emits <b>both</b> unconditionally — a dialect needing both ends (or neither) is expressible without
+    ///     touching the call site.</para>
+    /// </summary>
+    string SingleRowLimitSuffix { get; }
+
     /// <summary>Catalog query listing schemas — the browser's <c>RootAsync</c> level. Takes no parameters.</summary>
     string ListSchemasSql { get; }