fix(tmux): compose draft-transition with landed box detection (#1332, O1)

Rebased onto current next and composes the two sibling mechanisms per the
O1 ruling: the cursor-row draft transition stays the authoritative
runtime-agnostic delivered verdict; next's locate_input_box (two shapes:
prompt glyph, pi rule box) is adopted as positive DRAFT evidence only,
covering the cursor-row check's blind spot on redrawn TUIs that park the
cursor off the input line. Box-absence proves nothing and can never
produce UNDELIVERED: a shapeless-but-submitting pane delivers via the
cursor-row transition regardless.

The verdict suite gains fixtures 6 and 6b: the 2026-09-04 scratch probe
is the regression test (shapeless REPL that submits must report
delivered; shape probing alone exited 2 with retry advice on a consumed
message), plus the raw/no-echo shapeless guard arm. Measured limit
recorded in the suite: a shapeless cooked non-reading pane is
indistinguishable from a delivering one by any runtime-agnostic signal
available to the sender.

Rerun evidence: pipefail scanner 7/7, glyph-agnostic suite 6/6, verdict
suite 8/8, live shapeless probe rc=0 delivered with consumption proof.

packages/mosaic/framework/tools/tmux/test-send-message-verdict.sh

@@ -20,6 +20,12 @@
 #   5. DRAFT       — the same glyphless box, holding our tail across every flush
 #      (box shape)   Enter => exit 2, stderr "unsubmitted draft". Pre-#1362 this
 #                    also reported unconfirmed, so the true state was invisible.
+#   6. DELIVERED   — a SHAPELESS REPL (no glyph, no box) that submits => exit 0.
+#      The 2026-09-04 scratch probe regression: shape probing alone reports "may
+#      be UNDELIVERED" on this pane while the message is consumed; the cursor-row
+#      draft transition is the authoritative runtime-agnostic verdict.
+#   6b. UNCONFIRMED— a shapeless pane in raw/no-echo mode that never reads stdin
+#      (shapeless)   => exit 2 "could not confirm submission" (never delivered).
 set -uo pipefail
 
 HERE=$(cd -- "$(dirname -- "$0")" && pwd)
@@ -154,6 +160,51 @@ else
   fi
 fi
 
+# --- Fixtures 6 and 6b: a SHAPELESS REPL. The pane renders nothing at all: no
+#     prompt glyph and no rule box, so locate_input_box() alone can never see it
+#     and shape-probing alone reports "may be UNDELIVERED" on a delivered message
+#     (measured live 2026-09-04, scratch probe: a shapeless consumer CONSUMED the
+#     message while the shipped shape probe exited 2 with retry advice - the exact
+#     #1257 regression). The cursor-row draft transition is the authoritative,
+#     runtime-agnostic delivered verdict: fixture 6's consumer submits => exit 0.
+#     Fixture 6b is the guard arm: a shapeless pane whose foreground never reads
+#     stdin keeps the echoed paste on the cursor line across every flush Enter =>
+#     DRAFT => exit 2, never delivered.
+cat > "$TMP/shapeless.py" <<'SHAPELESS'
+import sys
+for line in sys.stdin:
+    pass  # consume and render nothing
+SHAPELESS
+tmux -L "$SOCKET" new-session -d -s shapeless -c "$TMP" "exec python3 -u '$TMP/shapeless.py'"
+sleep 0.3
+out=$("$SEND" -L "$SOCKET" -t "=shapeless" -m "fixture six shapeless consumed ok" 2>"$TMP/e6"); rc=$?
+if [ "$rc" -eq 0 ] && grep -qF "✓ delivered" <<<"$out"; then
+  ok "delivered: shapeless REPL that submits => exit 0 ✓ delivered (probe regression)"
+else
+  no "delivered: shapeless REPL that submits => exit 0 ✓ delivered" "rc=$rc out=[$out] err=[$(cat "$TMP/e6")]"
+fi
+
+# MEASURED LIMIT (2026-09-04, this suite's development): a shapeless pane in
+# COOKED mode whose foreground never reads stdin (e.g. 'sleep infinity') scrolls
+# its kernel echo off the cursor row on the flush Enter, so no runtime-agnostic
+# signal available to the sender distinguishes it from a delivering pane. The
+# non-reading guard therefore requires either a locatable box still carrying the
+# tail (fixture 3) or raw/no-echo mode (fixture 2b). Real REPL seats read stdin,
+# which is why this limit is not reachable against agent seats; recorded here so
+# nobody rediscovers it as a silent gap.
+tmux -L "$SOCKET" new-session -d -s shapelessraw -c "$TMP" 'stty raw -echo; exec sleep infinity'
+sleep 0.3
+if out=$("$SEND" -L "$SOCKET" -t "=shapelessraw" -r 1 -m "fixture six b shapeless raw never consumed" 2>"$TMP/e6b"); then
+  no "unconfirmed: shapeless raw non-reading pane must NOT report success" "expected exit 2, got 0 (out=[$out])"
+else
+  rc=$?
+  if [ "$rc" -eq 2 ] && grep -qF "could not confirm submission" "$TMP/e6b"; then
+    ok "unconfirmed: shapeless raw non-reading pane => exit 2 + 'could not confirm submission'"
+  else
+    no "unconfirmed: shapeless raw non-reading pane => exit 2 + stderr" "rc=$rc err=[$(cat "$TMP/e6b")]"
+  fi
+fi
+
 echo "---"
 echo "PASS=$PASS FAIL=$FAIL"
 [ "$FAIL" -eq 0 ]