Versioning: INBUXA's own dated version, with the Stalwart base shown

inbuxa --version, the banner, startup events, OpenTelemetry and the JMAP
implementation string read "2026.9.18 (Stalwart 0.16.22)". The base comes
from Cargo, which keeps following upstream so version bumps merge cleanly.
Received headers and IMAP ID carry the INBUXA version.

crates/types/src/branding.rs

@@ -64,3 +64,25 @@ pub fn env_var(name: &str) -> Result<String, std::env::VarError> {
         found => found,
     }
 }
+
+/// INBUXA's own version, dated like the rest of its family: `YYYY.M.D`, with
+/// a letter or `.N` suffix for a second release on one day. It's set here and
+/// not in Cargo.toml, so upstream's version bumps merge without conflicts.
+#[macro_export]
+macro_rules! brand_version {
+    () => {
+        "2026.9.18"
+    };
+}
+
+/// The version with the Stalwart release it's built on, e.g.
+/// `2026.9.18 (Stalwart 0.16.22)`. The base comes from Cargo, which follows
+/// upstream, so it's always the base actually compiled in. It matters because
+/// Stalwart's data upgrades are one-way. Once INBUXA stops tracking upstream,
+/// this becomes just the version.
+#[macro_export]
+macro_rules! brand_version_full {
+    () => {
+        concat!($crate::brand_version!(), " (Stalwart ", env!("CARGO_PKG_VERSION"), ")")
+    };
+}