docs: correct solo-mission count 21 -> 20 (level_54 'Cuba' is the ending-credits sequence, not a mission)

The campaign has 20 playable missions (SP_LEVEL_MAX // 20 in
bondconstants.h; mission_folder_setup_entries in front.c has exactly 20
entries). The old '21' counted -level_54 (LEVELID_CUBA), which the front
end jumps to after the final mission for the extended cast display +
credits scroll (do_extended_cast_display / bondviewRenderCredits, D129) -
it is not a selectable mission.

- README, docs/index.md, .github/release-notes.md,
  docs/security-and-fidelity-status.md: 21 -> 20 in current-status claims
- AGENTS.md phase status: same correction
- docs/dev/LEVEL-STATUS.md + LEVEL-PLAYTEST.md: relabel the level list;
  counting note explains historical '21/21 PASS' sweep results = 21 stage
  boots (20 missions + ending sequence)
- social-preview card: stat line 21 -> 20, PNG regenerated (1280x640)

README.md

@@ -73,9 +73,10 @@ listed plainly there.
 ## Status
 
 **v0.3.0 - playable, with known rough edges.** The full single-player
-campaign is completable end to end (all 21 missions, Agent difficulty,
-playtested), at a steady 60 fps; all 21 solo missions load, render and run
-crash-free, verified on Windows, Linux and real Steam Deck hardware. Feedback
+campaign is completable end to end (all 20 missions, Agent difficulty,
+playtested), at a steady 60 fps; all 20 solo missions — plus the
+end-of-campaign credits sequence — load, render and run crash-free, verified
+on Windows, Linux and real Steam Deck hardware. Feedback
 is very welcome.
 
 What a release actually installs (no networking, no telemetry, no ROM or
@@ -84,8 +85,8 @@ game's logic: [Security & fidelity status](docs/security-and-fidelity-status.md)
 
 **Working:** boot sequence and front end (menu → mission select → briefing →
 start), front-end menu navigation on the left stick to match the F10 overlay
-(D282); all 21 solo missions load, render and are crash-free (full campaign
-playtested end to end at Agent difficulty); steady 60 fps
+(D282); all 20 solo missions load, render and are crash-free (full campaign
+playtested end to end at Agent difficulty, including the ending sequence); steady 60 fps
 (software RSP off the presentation critical path); full audio: in-level music and SFX; keyboard + mouse (click-to-lock, proportional aim mode, a single
 simplified sensitivity control) and a modern dual-stick controller layout;
 automatic widescreen FOV scaling; Bond is fixed in cutscenes (no more