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/diff_words.py

@@ -0,0 +1,74 @@
+#!/usr/bin/env python3
+"""Find guest addresses holding one value in snapshot A and another in snapshot B.
+
+The classic differential search, which is how you locate a field whose ADDRESS is
+unknown but whose VALUE is known at two moments. Used here for the GamePart slot:
+`GP_EXTRAS` = 5 on the extras screen, `GP_MISSION_SELECT` = 7 on the next one, so
+the slot is a word that reads 5 then 7.
+
+Both snapshots are sparse copies of `/dev/shm/xenia_memory_*`, so the ~4.6 GB of
+holes cost nothing: walk A's allocated extents with SEEK_DATA and only look at B
+where A already matched.
+
+    diff_words.py <snapA> <valA> <snapB> <valB> [--near <va> <span>]
+"""
+import os
+import struct
+import sys
+
+sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
+from gmem import off_to_vas  # noqa: E402
+
+
+def main():
+    if len(sys.argv) < 5:
+        print(__doc__)
+        return 1
+    pa, va_, pb, vb_ = sys.argv[1], int(sys.argv[2], 0), sys.argv[3], int(sys.argv[4], 0)
+    pat_a = struct.pack(">I", va_)
+    fa, fb = open(pa, "rb"), open(pb, "rb")
+    size = min(os.path.getsize(pa), os.path.getsize(pb))
+
+    hits, scanned = [], 0
+    off = 0
+    while off < size:
+        try:
+            off = os.lseek(fa.fileno(), off, os.SEEK_DATA)
+        except OSError:
+            break
+        if off >= size:
+            break
+        try:
+            end = min(os.lseek(fa.fileno(), off, os.SEEK_HOLE), size)
+        except OSError:
+            end = size
+        while off < end:
+            n = min(1 << 22, end - off)
+            fa.seek(off)
+            buf = fa.read(n)
+            scanned += len(buf)
+            i = 0
+            while True:
+                i = buf.find(pat_a, i)
+                if i < 0:
+                    break
+                p = off + i
+                if p % 4 == 0:
+                    fb.seek(p)
+                    if fb.read(4) == struct.pack(">I", vb_):
+                        hits.append(p)
+                i += 1
+            off += n
+
+    print(f"scanned {scanned/1e6:.0f} MB of allocated guest memory")
+    print(f"addresses reading {va_} in A and {vb_} in B: {len(hits)}")
+    for h in hits[:40]:
+        vas = off_to_vas(h)
+        print(f"   file off {h:#012x}   guest VA {[hex(v) for v in vas][:2]}")
+    if len(hits) > 40:
+        print(f"   … and {len(hits)-40} more")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())