Pack: pack-dist.ps1 -- assemble a runnable Steam test dist (Phase 7)

Adapt RP412's pack-dist.ps1 for BattleTech. Assembles dist\ from a Release
Steam build (build-steam\Release\btl4.exe -- Release so it runs on a machine
without VS's non-redistributable debug CRT) plus steam_api.dll + OpenAL32.dll,
the content\ data (AUDIO/GAUGE/VIDEO, BTL4.RES, BTDPL.INI, the VREND
manifests, bindings.txt, DEV/MP/MP1 eggs), oalinst.exe (OpenAL fallback),
tools\btconsole.py, a TEST-only steam_appid.txt=480, a self-documenting
environ.ini (BT412STEAM=1 + the L4MFDSPLIT cockpit), start.bat, and a README
that states the multiplayer status plainly (lobby works; the host->member
mission feed is deferred; classic btconsole.py TCP path works today). -Zip
emits BattleTech-4.12.zip (~46 MB).

Verified: the Release exe boots a live DEV.EGG mission (optimizer-clean), and
the packaged exe run from dist\ brings the Steam transport up (FakeIP
allocated, lobby available) -- self-contained. dist/ + the zip are gitignored.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -1,6 +1,10 @@
 # build output
 /build/
 /build-steam/
+
+# packaged distribution (pack-dist.ps1 output) + its zip
+/dist/
+/BattleTech-*.zip
 *.obj
 *.pdb
 *.ilk