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)

docs/security-and-fidelity-status.md

@@ -82,7 +82,8 @@ port of the N64's PRNG, but wasn't — a shift-operation helper didn't match
 the real MIPS64 instruction semantics it was meant to mirror. This was
 independently confirmed by hand-tracing the assembly against the C port,
 tracked as finding D284, and fixed before the release (verified clean
-across a full 21-level sweep, no crashes). It affected loot placement, AI
+across a full level sweep — all 20 missions plus the ending sequence —
+no crashes). It affected loot placement, AI
 behavior variance, and replay-state determinism from the very first random
 draw, in every release before v0.3.0. Because of the fix, RNG-derived
 output — including the save-file CRC — changed, which meant existing save