port: audit the handoff -- four stale statements, a broken link, and the

residue in one place

Every MISSION row is answered, so this iteration checked the deliverable
itself rather than opening new scope: an answer that is not reachable
from HANDOFF.md has not been delivered, and I have been editing that page
by surgical replacement for a dozen iterations.

Four things were wrong or stale:

- the Q5 row still said initial focus was 2x TUTORIAL, 1x NEW GAME while
  its own bullet said four boots -- I updated the body last time and not
  the row;
- the Q2 bullet still described the other four main-menu destinations as
  "likely by name -- an inference, not a measurement", which Q4 has since
  MEASURED, and it listed GP_MISSION_SELECT as a main-menu destination
  when MISSION SELECT lives under EXTRAS;
- the Q1 bullet claimed that if the game presents at 60 Hz "nothing else
  on this page changes". That is now false: Q7's declared fade-in lengths
  are keyframe-derived and would halve. The same bullet also missed that
  the transition capture is a SECOND independent measurement supporting
  1 unit = 1/60 s -- the quad is declared black for 12 units, 0.20 s
  under the conversion, and the capture measured the plateau at
  0.17-0.23 s. Recorded as corroboration, not proof, because the plateau
  spans two screens' fades and cannot separate them;
- bgm-two-stems.md linked ../port/MISSION.md from inside structures/,
  which resolves nowhere.

Also named the two videos actually in scope, since "3.3 GB of video, two
files are in scope" is not much use without the filenames.

And added a What Is Still Open section. With every row green the residue
was scattered across eight pages; it is now one table with why each item
is stuck. Two entries share a single unblocker -- an emulator whose audio
path can be observed -- and they are the only ones that would change a
port decision.

docs/re/structures/bgm-two-stems.md

@@ -5,7 +5,7 @@ over all 32 BGM banks. 🟡 for the *role* of the second wave — **measured** b
 signal analysis, and the two readings that survive are named below. ❔ which bank
 is the menu's music is **undecodable from the disc**, with the reach stated.
 
-Answers [MISSION Q10](../port/MISSION.md). It also **withdraws the premise**: the
+Answers [MISSION Q10](../../port/MISSION.md). It also **withdraws the premise**: the
 handoff said `BGM_001.slb` is *three* sub-waves of 10 KB / 4.47 MB / 4.67 MB. The
 10 KB is the bank **header**, not a wave.