feat(config): ARCHIPELAGO_USE_QUADLET_BACKENDS env override

Adds an env-var lever for Phase 3.2's use_quadlet_backends flag so the
20× harness can flip the path on per-node without a config.json edit
(which would require an archipelago.service restart — and that triggers
FM3 cgroup cascade until Phase 3.5 ships, so we can't ask anyone to
reconfigure live nodes that way today).

Truthy parsing centralised in `parse_truthy_env` (1, true, yes, on —
case-insensitive, whitespace-trimmed). Anything else is false. The
helper is unit-tested so future env-var flags can reuse the same shape.

Also adds a default-off regression test for use_quadlet_backends so
flipping the default ahead of the 20× verification fires immediately.

TESTING.md documents the Environment= snippet for the systemd drop-in
so the next operator can flip the flag on a debug node without
re-deriving the recipe.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

core/archipelago/src/config.rs

@@ -182,6 +182,17 @@ impl Config {
             config.nostr_tor_proxy = if s.is_empty() { None } else { Some(s) };
         }
 
+        // Phase 3.2 of v1.7.52. Truthy values (1, true, yes, on — case-insensitive)
+        // route backend installs through the Quadlet path without requiring a
+        // config.json edit + archipelago.service restart (which would trigger
+        // FM3 cgroup cascade until 3.5 ships). Anything else (or unset) leaves
+        // the config.json value untouched.
+        if let Ok(v) = std::env::var("ARCHIPELAGO_USE_QUADLET_BACKENDS") {
+            if parse_truthy_env(&v) {
+                config.use_quadlet_backends = true;
+            }
+        }
+
         // Host IP for container env vars (detect if not set)
         if let Ok(ip) = std::env::var("ARCHIPELAGO_HOST_IP") {
             config.host_ip = ip;
@@ -234,6 +245,18 @@ impl Default for Config {
     }
 }
 
+/// Recognise the canonical "the user meant true" forms for boolean env
+/// vars: 1, true, yes, on (case-insensitive, surrounding whitespace
+/// trimmed). Anything else — including the typo'd "ture" or the empty
+/// string — counts as false. Centralised so future env flags stay
+/// consistent with each other.
+fn parse_truthy_env(raw: &str) -> bool {
+    matches!(
+        raw.trim().to_ascii_lowercase().as_str(),
+        "1" | "true" | "yes" | "on"
+    )
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -437,4 +460,24 @@ mod tests {
         assert!(!config.nostr_relays.is_empty());
         assert!(config.nostr_relays.iter().all(|r| r.starts_with("wss://")));
     }
+
+    #[test]
+    fn parse_truthy_env_recognises_canonical_forms() {
+        for t in ["1", "true", "TRUE", "yes", "Yes", "on", "ON", "  true  "] {
+            assert!(parse_truthy_env(t), "{t:?} should parse truthy");
+        }
+        for f in ["", "0", "false", "no", "off", "ture", "anything else", " "] {
+            assert!(!parse_truthy_env(f), "{f:?} should NOT parse truthy");
+        }
+    }
+
+    #[test]
+    fn test_config_use_quadlet_backends_defaults_off() {
+        // Phase 3.2 of v1.7.52 — the new path stays gated until the 20×
+        // harness goes green on .228 and .198. Flipping this default
+        // ahead of that would route every backend install through code
+        // we haven't fleet-validated yet.
+        let config = Config::default();
+        assert!(!config.use_quadlet_backends);
+    }
 }