feat(execution_trace): literal-100% residue — dichotomy wave

Port the last audit-missing fn name-preserving: _journal_path, the
facade HOME-seam twin (expanduser env-read stays facade; the twin is
the posixpath join law over its output — exactly one separating
slash). Truth +5 journal_path cases (root-slash/empty/relative/
trailing-slash HOME quirks frozen from the live oracle); existing
groups byte-stable. Battery 4/4 green incl. replay_journal_path;
live parity 820/820; clippy -D warnings clean; adjacent pytest 9/9
BOTH pins. Fn coverage 3/4 -> 4/4.

scripts/gen_execution_trace_truth.py

@@ -163,12 +163,41 @@ case_trace("missing-journal-with-record", "t9", {
 }, None)
 case_trace("empty-record-falsy-law", "t2", {}, JOURNAL)
 
+# ── _journal_path: HOME seam + posixpath join law ──────────────────────────
+# The oracle reads expanduser (HOME env); each case patches HOME, captures
+# BOTH the expanduser output (the twin's input) and the oracle's full path.
+cases_jp = []
+
+
+def case_jp(name, home):
+    import posixpath
+    old = os.environ.get("HOME")
+    os.environ["HOME"] = home
+    try:
+        expanded = posixpath.expanduser("~/.rune")
+        got = et._journal_path()
+    finally:
+        if old is None:
+            os.environ.pop("HOME", None)
+        else:
+            os.environ["HOME"] = old
+    cases_jp.append({"name": name, "home": home, "input": expanded,
+                     "expect": got})
+
+
+case_jp("home-normal", "/home/nick")
+case_jp("home-root-slash-double-slash-quirk", "/")
+case_jp("home-empty-string", "")
+case_jp("home-relative", "rel/base")
+case_jp("home-trailing-slash", "/x/")
+
 truth = {
     "module": "execution_trace",
     "core_pin": "RUNE_ET_CORE=python",
     "build_receipt_cases": cases_build,
     "read_outcome_events_cases": cases_read,
     "trace_task_cases": cases_trace,
+    "journal_path_cases": cases_jp,
 }
 
 out = REPO / "src" / "rust" / "execution_trace" / "frozen_et_truth.json"

src/rust/execution_trace/frozen_et_truth.json

@@ -188,6 +188,38 @@
   }
  ],
  "core_pin": "RUNE_ET_CORE=python",
+ "journal_path_cases": [
+  {
+   "expect": "/home/nick/.rune/journal/events.jsonl",
+   "home": "/home/nick",
+   "input": "/home/nick/.rune",
+   "name": "home-normal"
+  },
+  {
+   "expect": "/.rune/journal/events.jsonl",
+   "home": "/",
+   "input": "/.rune",
+   "name": "home-root-slash-double-slash-quirk"
+  },
+  {
+   "expect": "/.rune/journal/events.jsonl",
+   "home": "",
+   "input": "/.rune",
+   "name": "home-empty-string"
+  },
+  {
+   "expect": "rel/base/.rune/journal/events.jsonl",
+   "home": "rel/base",
+   "input": "rel/base/.rune",
+   "name": "home-relative"
+  },
+  {
+   "expect": "/x/.rune/journal/events.jsonl",
+   "home": "/x/",
+   "input": "/x/.rune",
+   "name": "home-trailing-slash"
+  }
+ ],
  "module": "execution_trace",
  "read_outcome_events_cases": [
   {

src/rust/execution_trace/src/branch_battery.rs

@@ -3,7 +3,7 @@
 //! against the LIVE Python oracle with RUNE_ET_CORE=python pinned and
 //! deep-copied inputs).
 
-use crate::{build_receipt, read_outcome_events, trace_task};
+use crate::{_journal_path, build_receipt, read_outcome_events, trace_task};
 use evidence_provenance::pyjson::{self, Json};
 
 const TRUTH: &str = include_str!("../frozen_et_truth.json");
@@ -108,6 +108,18 @@ fn replay_read_outcome_events() {
     }
 }
 
+#[test]
+fn replay_journal_path() {
+    for c in cases("journal_path_cases") {
+        let got = Json::Str(_journal_path(&jstr(at(&c, &["input"]))));
+        assert_case(
+            &jstr(at(&c, &["name"])),
+            at(&c, &["expect"]).expect("expect"),
+            &got,
+        );
+    }
+}
+
 #[test]
 fn replay_trace_task() {
     for c in cases("trace_task_cases") {

src/rust/execution_trace/src/lib.rs

@@ -184,6 +184,16 @@ pub fn trace_task(task_id: &str, task_record: Option<&Json>, outcomes: Option<&J
 
 // ── FFI (doc-envelope) ───────────────────────────────────────────────────
 
+/// Name-preserving twin of the facade's `_journal_path`: the journal
+/// lives at <expanduser("~/.rune")>/journal/events.jsonl. The
+/// expanduser env-read stays in the facade (no-env kernel law) — this is
+/// the os.path.join law over its output: exactly one separating slash,
+/// none added when the base already ends with one.
+pub fn _journal_path(home: &str) -> String {
+    let sep = if home.ends_with('/') { "" } else { "/" };
+    format!("{home}{sep}journal/events.jsonl")
+}
+
 fn envelope(result: Result<Json, (String, String)>) -> *mut c_char {
     let body = match result {
         Ok(value) => obj(vec![("ok", Json::Bool(true)), ("value", value)]),