fix(fold): key the managed Lua widening on the EFFECTIVE edit site

PR #149 review round 5, finding 1 — correct, and both round-4 bugs did
survive through the intercept path.

Round 4 moved the widening off the point and onto `edit_start_of(&op)`,
but ran it in `run_buffer_edit` BEFORE `run_managed_edit`. A managed
buffer intercept may legally rewrite `pos` / `start` / `end`
(`LuaInterceptView::intercept_edit`), so the requested op is not where
the edit lands:

  - requested outside -> intercept relocates inside: the edit stayed
    hidden;
  - requested inside -> intercept relocates outside: an unrelated fold
    opened.

The seam still covers BOTH paths — hooking only `run_managed_edit` would
let an interactive `bypass_intercept` edit escape, which is why the
framing put it on the common entry — but each path now keys on its own
effective site:

  - `run_bypass_edit` applies its op verbatim, so `run_buffer_edit`
    hooks it there;
  - `run_managed_edit` hooks after the intercept chain settles and
    before the apply, on the op the chain returned. A chain that raises
    applies nothing, so it unfolds nothing.

The registry borrow is released at that point, and the helper reads only
`SharedCore` + the fold registry, so no borrow conflicts with phase 3.

Three new tests, all through real `M-x` with a real
`pmacs.buffer.add_intercept`: relocate-into-a-fold must unfold,
relocate-out-of-a-fold must not, and a rejected chain must not. Each
asserts the buffer text first, so the test fails loudly if the intercept
stops relocating rather than silently passing.

Suite 45 -> 48.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Q5BkezMppbpCgGAYk2ftxV

src/lua_bindings/mod.rs

@@ -1309,12 +1309,16 @@ fn run_buffer_edit(
     bypass_intercept: bool,
 ) -> mlua::Result<crate::rope::Edit> {
     // Arc 6 Stage 2 (Q#FD19): the interactive-Lua-command unfold seam.
-    // Hooked HERE — above both `run_managed_edit` and `run_bypass_edit` —
-    // so an interactive command that passes `bypass_intercept` does not
-    // escape the widening. Keyed on where the edit LANDS, read before
-    // `op` is consumed below.
-    unfold_before_interactive_lua_edit(lua, id, edit_start_of(&op));
+    // BOTH paths are covered — hooking only `run_managed_edit` would let
+    // an interactive `bypass_intercept` edit escape — but each keys on
+    // *its own* effective edit site (round-5 F1):
+    //
+    // - bypass applies `op` verbatim, so the site is known right here;
+    // - managed runs the intercept chain first, and an intercept may
+    //   legally relocate `pos` / `start` / `end`, so only the op the
+    //   chain settles on names where the edit will land.
     if bypass_intercept {
+        unfold_before_interactive_lua_edit(lua, id, edit_start_of(&op));
         run_bypass_edit(lua, id, op)
     } else {
         run_managed_edit(lua, id, op)
@@ -1359,6 +1363,12 @@ fn edit_start_of(op: &EditOp<'_>) -> u64 {
 /// [`apply_active_edit`](crate::editor_core::EditorCore::apply_active_edit)'s
 /// funnel keys on the point; only this Lua path can diverge.
 ///
+/// "Edit site" means the **effective** one (round-5 F1). On the managed
+/// path a buffer intercept may legally rewrite `pos` / `start` / `end`
+/// before the op is applied, so [`run_managed_edit`] calls this *after*
+/// the intercept chain settles; only [`run_bypass_edit`], which applies
+/// its op verbatim, can name the site up front.
+///
 /// Matches Stage 1's data-API exemption: `pmacs.buffer.insert` called
 /// from a plugin, a hook, or a bare Lua chunk unfolds nothing.
 fn unfold_before_interactive_lua_edit(lua: &Lua, id: BufferId, edit_start: u64) {
@@ -1435,6 +1445,19 @@ fn run_managed_edit(lua: &Lua, id: BufferId, op: EditOp<'_>) -> mlua::Result<cra
         Ok(current)
     })();
 
+    // Arc 6 Stage 2 (Q#FD19, round-5 F1): the interactive unfold keys on
+    // the EFFECTIVE edit site, so it must run here — after the chain has
+    // settled, before the apply. An intercept may legally relocate `pos`
+    // / `start` / `end`, so widening on the *requested* op would leave a
+    // relocated-into-a-fold edit invisible, and would open an unrelated
+    // fold when the intercept moved the edit out of one. A rejected
+    // chain (`Err`) applies nothing, so it unfolds nothing. The registry
+    // borrow is released at this point; this reads `SharedCore` and the
+    // fold registry only.
+    if let Ok(final_op) = intercept_result.as_ref() {
+        unfold_before_interactive_lua_edit(lua, id, edit_start_of(final_op));
+    }
+
     // Phase 3: re-borrow, restore views, clear mid-edit flag, apply.
     // We restore views and clear the flag even on intercept error,
     // so the buffer is left in a usable state.

tests/folding_stage2_acceptance.rs

@@ -1180,6 +1180,118 @@ fn an_interactive_edit_outside_the_fold_leaves_it_closed() {
     );
 }
 
+/// Attach a buffer intercept that relocates every `insert` to `to`.
+/// This is a supported, documented rewrite (`pos` / `start` / `end` may
+/// be overridden), and it is what makes the *requested* op's position an
+/// unreliable answer to "where will this edit land?".
+fn relocate_inserts_to(s: &EditorState, to: u64) {
+    exec(
+        s,
+        &format!(
+            r#"
+            pmacs.buffer.add_intercept(pmacs.window.buffer(), function(op)
+              if op.kind == "insert" then
+                return {{ kind = "insert", pos = {to} }}
+              end
+              return nil
+            end)
+            "#
+        ),
+    );
+}
+
+#[test]
+fn an_intercept_relocating_the_edit_into_a_fold_unfolds_it() {
+    // Round-5 F1. The command requests an edit OUTSIDE the fold; a
+    // managed intercept moves it INSIDE. Keying on the requested op
+    // would leave what the user just wrote invisible.
+    let (s, _id) = seeded();
+    let mut s = s;
+    fold_active(&s, 2, 5); // head 2, hidden 3..=5 — fold range (11, 23]
+    relocate_inserts_to(&s, end_of(4)); // 19, inside the fold
+    set_cursor(&s, 0);
+    define(
+        &s,
+        "test.relocate_in",
+        "pmacs.window.buffer():insert(0, 'y')",
+    );
+    m_x(&mut s, "test.relocate_in");
+
+    let text: String = eval(
+        &s,
+        "local b = pmacs.window.buffer(); return b:slice(0, b:len())",
+    );
+    assert!(
+        text.starts_with("L00\nL01\nL02\nL03\nL04y"),
+        "the intercept really relocated the insert: {text:?}"
+    );
+    assert_eq!(
+        fold_count(&s),
+        0,
+        "the EFFECTIVE edit site was inside the fold, so it must be revealed"
+    );
+}
+
+#[test]
+fn an_intercept_relocating_the_edit_out_of_a_fold_leaves_it_closed() {
+    // The other direction: requested INSIDE, intercept moves it OUTSIDE.
+    // Keying on the requested op would open a fold nothing was written to.
+    let (s, _id) = seeded();
+    let mut s = s;
+    fold_active(&s, 2, 5);
+    relocate_inserts_to(&s, 0); // outside the fold
+    set_cursor(&s, 0);
+    define(
+        &s,
+        "test.relocate_out",
+        &format!("pmacs.window.buffer():insert({}, 'y')", end_of(4)),
+    );
+    m_x(&mut s, "test.relocate_out");
+
+    let text: String = eval(
+        &s,
+        "local b = pmacs.window.buffer(); return b:slice(0, b:len())",
+    );
+    assert!(
+        text.starts_with("yL00"),
+        "the intercept really relocated the insert: {text:?}"
+    );
+    assert_eq!(
+        fold_count(&s),
+        1,
+        "nothing landed inside the fold, so it must stay closed"
+    );
+}
+
+#[test]
+fn a_rejected_intercept_chain_unfolds_nothing() {
+    // A chain that raises applies no edit at all, so there is nothing to
+    // reveal — the widening must not fire on a request that never lands.
+    let (s, _id) = seeded();
+    let mut s = s;
+    fold_active(&s, 2, 5);
+    exec(
+        &s,
+        r#"
+        pmacs.buffer.add_intercept(pmacs.window.buffer(), function(op)
+          if op.kind == "insert" then error("nope") end
+          return nil
+        end)
+        "#,
+    );
+    set_cursor(&s, 0);
+    define(
+        &s,
+        "test.rejected",
+        &format!(
+            "pcall(function() pmacs.window.buffer():insert({}, 'y') end)",
+            end_of(4)
+        ),
+    );
+    m_x(&mut s, "test.rejected");
+    assert_eq!(fold_count(&s), 1, "a rejected edit reveals nothing");
+}
+
 #[test]
 fn a_bypass_intercept_interactive_edit_also_unfolds() {
     // Pins the seam at `run_buffer_edit`, above the managed/bypass split: