Reorganise codebase: EffectiveConfig, parsers, tests; move tooltips to Metadata

Infrastructure:
- Parsers under Serialization/Parsers: Dnsmasq, DnsmasqConfig, EffectiveConfig
- EffectiveConfig: Abstractions, Metadata (keys, views, field builder, tooltips,
  placeholders, write semantics), Rendering, Editing, Validation/Handlers, Readonly
- Helpers/Config: only DnsmasqFileEncoding; DnsmasqOptionTooltips moved to Metadata
- Remove empty Infrastructure/Parsers and Helpers/Dnsmasq

Tests:
- Mirror main project: Serialization/Parsers (Dnsmasq, DnsmasqConfig),
  Services/Dnsmasq/Config, Services/EffectiveConfig, Models, Helpers (TestDataHelper)
- Namespaces updated; usings for TestDataHelper and Metadata where needed

Docs: README codebase overview and test layout updated.

src/DnsmasqWebUI/Infrastructure/Services/EffectiveConfig/Metadata/OptionValidationSemantics.cs

@@ -0,0 +1,63 @@
+using DnsmasqWebUI.Models.Dnsmasq.EffectiveConfig;
+
+namespace DnsmasqWebUI.Infrastructure.Services.EffectiveConfig.Metadata;
+
+/// <summary>
+/// Shared validation kinds for effective-config options.
+/// Describes the generic value shape; option-specific rules live in the semantic validator or handlers.
+/// </summary>
+public enum OptionValidationKind
+{
+    Flag,
+    Int,
+    String,
+    PathFile,
+    PathDirectory,
+    PathFileOrDirectory,
+    IpAddress,
+    HostOrIp,
+    KeyOnlyOrValue,
+    InversePair,
+    Complex
+}
+
+/// <summary>
+/// Whether a path value must exist (file/dir) or only its parent, for validation.
+/// </summary>
+public enum PathExistencePolicy
+{
+    None,
+    MustExist,
+    ParentMustExist
+}
+
+/// <summary>
+/// Reusable validation metadata for an option: validation kind, path policy, severity, and empty handling.
+/// </summary>
+public sealed record OptionValidationSemantics
+{
+    public OptionValidationKind Kind { get; }
+    public FieldIssueSeverity Severity { get; }
+    public bool AllowEmpty { get; }
+    public PathExistencePolicy? PathPolicy { get; }
+
+    public OptionValidationSemantics(
+        OptionValidationKind kind,
+        FieldIssueSeverity severity = FieldIssueSeverity.Error,
+        bool allowEmpty = true,
+        PathExistencePolicy? pathPolicy = null)
+    {
+        if (pathPolicy is not null && kind is not (
+            OptionValidationKind.PathFile or
+            OptionValidationKind.PathDirectory or
+            OptionValidationKind.PathFileOrDirectory))
+        {
+            throw new ArgumentException("PathPolicy can only be used with path validation kinds.", nameof(pathPolicy));
+        }
+
+        Kind = kind;
+        Severity = severity;
+        AllowEmpty = allowEmpty;
+        PathPolicy = pathPolicy;
+    }
+}