re(challenge): the part id is never persisted -- differential search says stack only

No literal 26 exists anywhere, so the GamePart id is computed. That does not stop
it being found: the id is KNOWN at each screen from the GamePart table (EXTRAS = 5,
MISSION SELECT = 7), so snapshot both and intersect. New tool diff_words.py does
the classic differential search over the sparse guest image, and find_partslot.sh
drives the two screens and runs it.

Result: 171 MB scanned, exactly 4 addresses read 5 then 7 -- 0x708FFBEC,
0x708FFCBC, 0x708FFDAC, 0x708FFE20 -- and all four are guest STACK (the same run's
log puts thread stacks at 0x709...). So the requested part id exists only as a
stack argument in flight; there is no persistent field, which is consistent with
finding no literal store, and means there is nothing stable to poke.

That closes the last memory-and-menu route to the challenge missions. Reaching
them needs either the genuine in-game unlock (an in-mission attainment, per
AVSCRIPT_COMMAND_ATTAINMENT_CHALLENGE_MISSION_CARGO_SCORE) or an emulator-side
hook that forces the transition -- a code change, not a poke.

diff_words.py is worth keeping well beyond this question: it locates any field
whose address is unknown but whose value is known at two moments.

tools/re-capture/find_partslot.sh

@@ -0,0 +1,69 @@
+#!/usr/bin/env bash
+# Locate the GamePart "slot" in guest memory by DIFFERENTIAL SEARCH.
+#
+# sub_821749C0 creates a part from slot+12 (the requested part id) and stores the
+# result at *(slot+16). The id is computed from a menu selection -- no literal 26
+# exists anywhere -- so it cannot be found statically. But the id is KNOWN at each
+# screen from the GamePart table (docs/re/challenge-mission-gate.md section 3):
+#
+#     GP_EXTRAS = 5      GP_MISSION_SELECT = 7
+#
+# so the slot is simply the word that reads 5 on the EXTRAS screen and 7 on the
+# MISSION SELECT screen. Snapshot both, intersect, and the candidates are few.
+#
+# Usage: find_partslot.sh [boot_timeout_s]
+set -u
+export HOME=/sylph-home/re DISPLAY=:99 SDL_AUDIODRIVER=dummy
+export XENIA_PAD_FILE=/tmp/xenia_pad.txt
+HERE="$(cd "$(dirname "$0")" && pwd)"
+pad()  { python3 "$HERE/pad.py"   "$@"; }
+poke() { python3 "$HERE/gpoke.py" "$@"; }
+BOOT_TIMEOUT="${1:-400}"
+say()  { echo "[$(date +%H:%M:%S)] $*"; }
+px()   { convert /tmp/nav-probe.png -format \
+         "%[fx:int(255*p{$1}.r)] %[fx:int(255*p{$1}.g)] %[fx:int(255*p{$1}.b)]" info: 2>/dev/null; }
+at_menu() {
+  screenshot /tmp/nav-probe.png >/dev/null 2>&1 || return 1
+  read -r r g b < <(px "648,221"); [ -n "${r:-}" ] || return 1
+  [ "$r" -gt 230 ] && [ "$g" -gt 230 ] && [ "$b" -gt 230 ] || return 1
+  read -r r2 g2 b2 < <(px "560,300"); [ -n "${r2:-}" ] || return 1
+  [ "$r2" -lt 120 ] && [ "$b2" -gt "$r2" ]
+}
+at_title() {
+  screenshot /tmp/nav-probe.png >/dev/null 2>&1 || return 1
+  read -r r g b < <(px "625,618"); [ -n "${g:-}" ] || return 1
+  [ "$g" -gt 130 ] && [ $((g - r)) -gt 45 ] && [ $((g - b)) -gt 45 ]
+}
+snap() { SHM=$(ls /dev/shm/xenia_memory_* 2>/dev/null | head -1); cp --sparse=always "$SHM" "$1"; }
+
+pkill -9 -x xenia_canary 2>/dev/null; sleep 1
+rm -f /dev/shm/xenia_* 2>/dev/null; : > "$XENIA_PAD_FILE"
+say "launching"
+run-canary --audio --apu=sdl --log_mask=13 \
+  --logged_profile_slot_0_xuid=E0300000EFBEA3D4 \
+  --hid=file --pad_file="$XENIA_PAD_FILE" &
+xsetroot -solid black 2>/dev/null || true
+
+MENU=0
+for i in $(seq 1 "$BOOT_TIMEOUT"); do
+  [ "$i" -lt 40 ] && { sleep 1; continue; }
+  if at_menu && sleep 1 && at_menu; then say "MAIN MENU after ${i}s"; MENU=1; break; fi
+  at_title && { say "  title — tapping A"; pad tap A 0.25; sleep 2; }
+  sleep 1
+done
+[ "$MENU" = 1 ] || { say "TIMEOUT"; exit 1; }
+
+say "-> EXTRAS (GamePart 5)"
+for _ in 1 2 3 4; do pad dpad down 0.06; sleep 0.35; done
+sleep 0.5; pad tap A 0.15; sleep 4
+screenshot "$HOME/shots/slot-01-extras.png" >/dev/null
+snap /sylph-home/re/snap-extras.bin; say "snapshot A (EXTRAS) taken"
+
+say "-> MISSION SELECT (GamePart 7)"
+pad tap A 0.15; sleep 5
+screenshot "$HOME/shots/slot-02-missionselect.png" >/dev/null
+snap /sylph-home/re/snap-msel.bin; say "snapshot B (MISSION SELECT) taken"
+
+say "intersecting: word == 5 in A and == 7 in B"
+python3 "$HERE/diff_words.py" /sylph-home/re/snap-extras.bin 5 /sylph-home/re/snap-msel.bin 7
+say "done — emulator left running"