flatten: move poc-memory contents to workspace root

No more subcrate nesting — src/, agents/, schema/, defaults/, build.rs
all live at the workspace root. poc-daemon remains as the only workspace
member. Crate name (poc-memory) and all imports unchanged.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/bin/find-deleted.rs

@@ -0,0 +1,56 @@
+// Find all deleted nodes that have no subsequent non-deleted version
+// (i.e., nodes that are currently dead).
+//
+// Also checks: is there a live node under the same key with a different UUID?
+// If not, the deletion was terminal — the node is gone.
+
+use std::collections::HashMap;
+use std::io::BufReader;
+use std::fs;
+use capnp::{message, serialize};
+use poc_memory::memory_capnp;
+use poc_memory::store::Node;
+
+fn main() {
+    let path = std::env::args().nth(1)
+        .unwrap_or_else(|| {
+            let dir = poc_memory::store::nodes_path();
+            dir.to_string_lossy().to_string()
+        });
+
+    let file = fs::File::open(&path).unwrap();
+    let mut reader = BufReader::new(file);
+
+    // Collect ALL entries, tracking latest version per key
+    let mut latest_by_key: HashMap<String, Node> = HashMap::new();
+    let mut all_entries = 0u64;
+
+    while let Ok(msg) = serialize::read_message(&mut reader, message::ReaderOptions::new()) {
+        let log = msg.get_root::<memory_capnp::node_log::Reader>().unwrap();
+        for node_reader in log.get_nodes().unwrap() {
+            all_entries += 1;
+            let node = Node::from_capnp_migrate(node_reader).unwrap();
+            let dominated = latest_by_key.get(&node.key)
+                .map(|n| node.version >= n.version)
+                .unwrap_or(true);
+            if dominated {
+                latest_by_key.insert(node.key.clone(), node);
+            }
+        }
+    }
+
+    // Find keys where the latest version is deleted
+    let mut dead: Vec<&Node> = latest_by_key.values()
+        .filter(|n| n.deleted)
+        .collect();
+    dead.sort_by(|a, b| a.key.cmp(&b.key));
+
+    eprintln!("Scanned {} entries, {} unique keys", all_entries, latest_by_key.len());
+    eprintln!("{} live nodes, {} deleted (terminal tombstones)\n",
+        latest_by_key.len() - dead.len(), dead.len());
+
+    for node in &dead {
+        println!("{:<60} v{:<4} {}b  prov={}",
+            node.key, node.version, node.content.len(), node.provenance);
+    }
+}