refactor: replace per-type merge() one-liners with a single generic template

Adds a constrained merge(T&, T&&) template in MergeReflectedType.hpp
that works for any type with MRDOCS_DESCRIBE_STRUCT. Non-template
overloads (DocComment, SourceInfo, TemplateInfo, ExtractionMode, etc.)
are still preferred by overload resolution.

This eliminates a bunch of merge() functions that did nothing but
forward to mergeReflected(), along with their MRDOCS_DECL declarations
in public headers. Three .cpp files that existed solely for those
one-liners are deleted entirely (Enum.cpp, EnumConstant.cpp,
NamespaceAlias.cpp).

Cleans up unused #include <mrdocs/Platform.hpp> from several .cpp files
that only needed it for MRDOCS_ASSERT in the deleted functions.

Author: gennaroprota

include/mrdocs/Metadata/Name.hpp

@@ -89,11 +89,6 @@ tag_invoke(
     tag_invoke(dom::ValueFromTag{}, v, *I, domCorpus);
 }
 
-/** Merge two Name metadata objects.
-*/
-void
-merge(Name& I, Name&& Other);
-
 } // mrdocs
 
 #endif

include/mrdocs/Metadata/Symbol/Concept.hpp

@@ -56,12 +56,6 @@ MRDOCS_DESCRIBE_STRUCT(
     (Template, Constraint)
 )
 
-/** Merge another ConceptSymbol into this one.
-*/
-MRDOCS_DECL
-void
-merge(ConceptSymbol& I, ConceptSymbol&& Other);
-
 /** Map a ConceptSymbol to a dom::Object.
 
     @param t The tag type.

include/mrdocs/Metadata/Symbol/Enum.hpp

@@ -81,14 +81,6 @@ allMembers(EnumSymbol const& T)
     return T.Constants;
 }
 
-/** Merge another EnumSymbol into this one.
-    @param I Destination symbol to update.
-    @param Other Source symbol providing data.
-*/
-MRDOCS_DECL
-void
-merge(EnumSymbol& I, EnumSymbol&& Other);
-
 /** Map a EnumSymbol to a dom::Object.
 
     @param t The tag type.

include/mrdocs/Metadata/Symbol/EnumConstant.hpp

@@ -43,14 +43,6 @@ MRDOCS_DESCRIBE_STRUCT(
     (Initializer)
 )
 
-/** Merge another EnumConstantSymbol into this one.
-    @param I Destination symbol to update.
-    @param Other Source symbol providing data.
-*/
-MRDOCS_DECL
-void
-merge(EnumConstantSymbol& I, EnumConstantSymbol&& Other);
-
 /** Map a EnumConstantSymbol to a dom::Object.
 
     @param t The tag type.

include/mrdocs/Metadata/Symbol/Friend.hpp

@@ -50,12 +50,6 @@ MRDOCS_DESCRIBE_STRUCT(
     (Type, id)
 )
 
-/** Merge another FriendInfo into this one.
-*/
-MRDOCS_DECL
-void
-merge(FriendInfo& I, FriendInfo&& Other);
-
 /** Merge friend declarations, deduplicating by symbol ID.
 
     @param dst The destination.

include/mrdocs/Metadata/Symbol/Function.hpp

@@ -164,14 +164,6 @@ MRDOCS_DESCRIBE_STRUCT(
      RefQualifier, Explicit, Attributes, FunctionObjectImpl)
 )
 
-/** Merge metadata from another function symbol.
-    @param I Destination symbol to update.
-    @param Other Source symbol providing additional data.
-*/
-MRDOCS_DECL
-void
-merge(FunctionSymbol& I, FunctionSymbol&& Other);
-
 /** Map a FunctionSymbol to a dom::Object.
 
     @param t The tag type.

include/mrdocs/Metadata/Symbol/Guide.hpp

@@ -67,14 +67,6 @@ MRDOCS_DESCRIBE_STRUCT(
     (Deduced, Template, Params, Explicit)
 )
 
-/** Merge another GuideSymbol into this one.
-    @param I Destination symbol to update.
-    @param Other Source symbol providing data.
-*/
-MRDOCS_DECL
-void
-merge(GuideSymbol& I, GuideSymbol&& Other);
-
 /** Map a GuideSymbol to a dom::Object.
 
     @param t The tag type.

include/mrdocs/Metadata/Symbol/Namespace.hpp

@@ -67,12 +67,6 @@ MRDOCS_DESCRIBE_STRUCT(
      Functions, Variables, Concepts, Guides, Usings)
 )
 
-/** Merge two tranches, appending members from the right-hand side.
-*/
-MRDOCS_DECL
-void
-merge(NamespaceTranche& I, NamespaceTranche&& Other);
-
 /** Join all tranche member lists into a single view.
     @return Lazy view spanning every category stored in the tranche.
 */
@@ -168,12 +162,6 @@ MRDOCS_DESCRIBE_STRUCT(
     (IsInline, IsAnonymous, UsingDirectives, Members)
 )
 
-/** Merge two namespaces, keeping existing members stable.
-*/
-MRDOCS_DECL
-void
-merge(NamespaceSymbol& I, NamespaceSymbol&& Other);
-
 /** View all members of the namespace across tranches.
     @return Lazy view across every member bucket.
 */

include/mrdocs/Metadata/Symbol/NamespaceAlias.hpp

@@ -47,12 +47,6 @@ MRDOCS_DESCRIBE_STRUCT(
     (AliasedSymbol)
 )
 
-/** Merge two alias symbols, preferring existing fields when present.
-*/
-MRDOCS_DECL
-void
-merge(NamespaceAliasSymbol& I, NamespaceAliasSymbol&& Other);
-
 /** Map a NamespaceAliasSymbol to a dom::Object.
 
     @param t The tag type.

include/mrdocs/Metadata/Symbol/Overloads.hpp

@@ -65,11 +65,6 @@ MRDOCS_DESCRIBE_STRUCT(
     (FuncClass, OverloadedOperator, Members, ReturnType)
 )
 
-/** Merge overload sets, preserving ordering in `Members`.
-*/
-MRDOCS_DECL
-void merge(OverloadsSymbol& I, OverloadsSymbol&& Other);
-
 /** Access the list of overload members.
     @return Reference to the ID list backing this set.
 */