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/EffectiveConfigWriteSemantics.cs

@@ -0,0 +1,41 @@
+namespace DnsmasqWebUI.Infrastructure.Services.EffectiveConfig.Metadata;
+
+/// <summary>
+/// How to serialize an option when writing the managed config file.
+/// This models write intent, not parse behavior.
+/// </summary>
+public enum EffectiveConfigWriteBehavior
+{
+    Flag,
+    SingleValue,
+    MultiValue,
+    KeyOnlyOrValue,
+    MultiKeyOnlyOrValue,
+    InversePair,
+}
+
+/// <summary>
+/// Source of truth for write semantics used by managed-config writing and readonly hints.
+/// Special options come from <see cref="EffectiveConfigSpecialOptionSemantics"/>; all others
+/// derive from parser behavior (Flag/Multi/LastWins).
+/// </summary>
+public static class EffectiveConfigWriteSemantics
+{
+    /// <summary>Returns write behavior for an option.</summary>
+    public static EffectiveConfigWriteBehavior GetBehavior(string optionName)
+    {
+        if (EffectiveConfigSpecialOptionSemantics.GetWriteBehavior(optionName) is { } special)
+            return special;
+        return EffectiveConfigParserBehaviorMap.GetBehavior(optionName) switch
+        {
+            EffectiveConfigParserBehavior.Flag => EffectiveConfigWriteBehavior.Flag,
+            EffectiveConfigParserBehavior.Multi => EffectiveConfigWriteBehavior.MultiValue,
+            _ => EffectiveConfigWriteBehavior.SingleValue,
+        };
+    }
+
+    /// <summary>Returns inverse-pair keys (enabled, disabled) for InversePair options; null otherwise.</summary>
+    public static (string KeyA, string KeyB)? GetInversePairKeys(string optionName) =>
+        EffectiveConfigSpecialOptionSemantics.GetInversePairKeys(optionName);
+}
+