Add the shared contracts both crates exchange: canonical boitier id
(digits only, no sign, no leading zero, non zero), MQTT topic names with
building and parsing (data, firstco, firstco-{id}, firstco_ACK,
maj_serv, maj_boitier_ACK), firstco payload and its ACK, maj_serv
configuration with range and version checks, maj_boitier_ACK, MPT v2
frame building next to its parser, public epoch validation, MQTT packet
size limits and a bounded backoff.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UH3JcaVWsmFLC6kyEsVL18

@@ -0,0 +14,4 @@
+    let mut tmp_name = file_name.to_os_string();
+    tmp_name.push(".tmp");
+    let tmp = path.with_file_name(tmp_name);
+    std::fs::write(&tmp, contents).with_context(|| format!("write {}", tmp.display()))?;

@@ -0,0 +175,4 @@
+        self.firstco.on_publish();
+        vec![
+            Outgoing::Publish {
+                topic: topics::FIRSTCO.to_string(),

@@ -0,0 +193,4 @@
+        }
+        if let Err(e) = mark_firstco_done(&self.paths.config) {
+            // Le serveur a provisionné le boîtier : rester en attente ferait
+            // republier firstco indéfiniment. Le prochain démarrage republiera.

@@ -0,0 +84,4 @@
+    pub fn validate_against(&self, applied: Option<u64>) -> Result<u64, ConfigRejection> {
+        let received = self.validate()?;
+        match applied {
+            Some(applied) if received < applied => {