proto: escape, moved from bxctl so gatewayd shows the same thing

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

crates/proto/src/escape.rs

@@ -0,0 +1,45 @@
+//! Text from outside (model output, tool arguments, server errors) made safe to show: control
+//! characters, zero-width and bidirectional marks, line separators and the BOM become `\uXXXX`.
+//! Used by `bxctl` for the terminal and by `gatewayd` for Mattermost, so both show the same.
+
+// The control bytes, the zero-width and Unicode line/paragraph separators, and the BOM. A code
+// point in any of these ranges must never reach the terminal raw.
+const ESCAPE_RANGES: &[(u32, u32)] = &[
+    (0x0000, 0x001f),
+    (0x007f, 0x009f),
+    (0x200b, 0x200f),
+    (0x2028, 0x202e),
+    (0x2060, 0x2069),
+    (0xfeff, 0xfeff),
+];
+
+pub fn escape_json_text(s: &str) -> String {
+    escape(s, false)
+}
+
+pub fn escape_model_text(s: &str) -> String {
+    escape(s, true)
+}
+
+fn escape(s: &str, model_text: bool) -> String {
+    let mut out = String::with_capacity(s.len());
+    for c in s.chars() {
+        let cp = u32::from(c);
+        if should_escape(c, model_text) {
+            out.push_str(&format!("\\u{:04x}", cp));
+        } else {
+            out.push(c);
+        }
+    }
+    out
+}
+
+// A control point is escaped, except that model text keeps \n and \t.
+fn should_escape(c: char, model_text: bool) -> bool {
+    let cp = u32::from(c);
+    if ESCAPE_RANGES.iter().any(|(lo, hi)| cp >= *lo && cp <= *hi) {
+        !(model_text && (cp == 0x09 || cp == 0x0a))
+    } else {
+        false
+    }
+}

crates/proto/src/lib.rs

@@ -3,6 +3,7 @@
 pub mod audit;
 pub mod chain;
 pub mod class;
+pub mod escape;
 pub mod frame;
 pub mod grant;
 pub mod hash;