memoryd: entries, and the check on every memory write

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

crates/memoryd/src/entries.rs

@@ -0,0 +1,44 @@
+//! The entries of `memory.md` and `user.md`: text separated by a line holding only `§` (M5a spec,
+//! section 3). A file's size is the characters of its entries joined by separator lines, which
+//! is what `render` writes, less the final newline.
+
+use proto::memory::SEPARATOR;
+
+/// The entries of a file's text: split at lines that are only `§` (spaces around it allowed),
+/// each trimmed, empty ones left out.
+pub fn parse(text: &str) -> Vec<String> {
+    let mut entries: Vec<String> = Vec::new();
+    let mut current: Vec<&str> = Vec::new();
+    for line in text.lines() {
+        if line.trim() == SEPARATOR {
+            push(&mut entries, &current);
+            current.clear();
+        } else {
+            current.push(line);
+        }
+    }
+    push(&mut entries, &current);
+    entries
+}
+
+fn push(entries: &mut Vec<String>, lines: &[&str]) {
+    let joined = lines.join("\n");
+    let trimmed = joined.trim();
+    if !trimmed.is_empty() {
+        entries.push(trimmed.to_string());
+    }
+}
+
+/// The entries joined by `\n§\n`, with a final newline; an empty list is an empty file.
+pub fn render(entries: &[String]) -> String {
+    if entries.is_empty() {
+        return String::new();
+    }
+    let joined = entries.join(&format!("\n{SEPARATOR}\n"));
+    format!("{joined}\n")
+}
+
+/// A file's size in characters, the one its limit is measured against.
+pub fn size(entries: &[String]) -> usize {
+    render(entries).trim_end_matches('\n').chars().count()
+}