Add standalone dynamic invoice Sieve rule

The prompts used to arrive at this change since the previous commit

User reported the Dynamisch kwartaal Roundcube filter is empty and asked for a new file containing a dynamic invoice-quarter filter while keeping the rest of the plan approved.

Any special observations that may be relevant for version management for this version. Be brief.

invoice_dynamic_quarter.sieve uses Date header year capture plus regex month tests and compiles with sievec. Existing untracked financ.sieve, codex.session, roundcube.txt, plan.md, and verplaats_log.json were left untouched.

invoice_dynamic_quarter.sieve

@@ -0,0 +1,40 @@
+# invoice_dynamic_quarter.sieve
+# Single Roundcube/Managesieve rule for dynamic invoice quarter routing.
+# Import this as a standalone filter set or copy the rule into an existing set.
+
+require ["fileinto", "date", "variables", "regex"];
+
+# rule:[Financieel / Facturen / Dynamisch kwartaal]
+if allof (
+    header :contains "Subject" [
+        "factuur",
+        "invoice",
+        "rekening",
+        "nota",
+        "betaalverzoek",
+        "receipt",
+        "orderbevestiging",
+        "order confirmation",
+        "betaling ontvangen",
+        "payment received",
+        "uw factuur",
+        "your invoice",
+        "je factuur"
+    ],
+    date :matches "Date" "year" "*"
+) {
+    set "jaar" "${1}";
+
+    if date :regex "Date" "month" "^(0?[123])$" {
+        set "kwartaal" "Q1";
+    } elsif date :regex "Date" "month" "^(0?[456])$" {
+        set "kwartaal" "Q2";
+    } elsif date :regex "Date" "month" "^(0?[789])$" {
+        set "kwartaal" "Q3";
+    } else {
+        set "kwartaal" "Q4";
+    }
+
+    fileinto "INBOX.Financieel.Facturen.${jaar}.${kwartaal}";
+    stop;
+}