Desert Strike — directional sprite and firing fixes v0.5.1

Fixes the helicopter appearing to fire from its tail. Original DOS sprite directions are now mapped north through east to south, with mirrored western views. Player tracers share the helicopter's visual height.

Also fixes reversed enemy headings, western bus/ATV mirroring, bus route-facing, Crotale and helicopter frame selection, and a tank direction incorrectly selecting a wreck. Turrets now use all 24 source direction frames.

Validation: 51 tests pass, including all four campaigns in both difficulties. Browser input tested all eight directions with cannon, Hydra and Hellfire (24 combinations), plus forward firing after turning in both classic control modes. Desktop and mobile control checks passed without uncaught errors. Complete browser campaign runs remain v0.5.0 evidence; this patch adds focused directional regressions.

The source sprites now agree with gameplay headings. Exact original-console animation cadence, flight/combat coefficients and overall difficulty remain unverified; see the fidelity ledger. Original asset credits remain in THIRD_PARTY.md.

For offline play, extract Desert-Strike-offline.zip and open Desert-Strike.html.