Track the airlock archive whole

A community build of 4.11 from another site. Three things have already
been taken out of it - RPL4.RES and GAUGE/L4GAUGE.CFG promoted into
assets/RP411 in f7c7000, and the console configs copied to
tools/console-config - and with the archive untracked none of that was
checkable. Keep it whole so it is.

Nothing here is read at build time or ships in a release; a README says
so, says what was promoted and why, and warns that rpl4opt.exe in this
folder is the community's 4.11 binary and not what BUILD.md produces.

It costs much less than the 111 MB on disk suggests: 767 of the 998 files
are byte-identical to assets/RP411 and git stores a blob once. The real
additions are the 223 WAVs, every one of which differs from ours, and
four audio files with no counterpart at all. Whether those WAVs are
better masters or just different renderings is unestablished, so they
stay unpromoted. Thumbs.db was already covered by .gitignore.

assets/airlock_RP411/README.md

@@ -0,0 +1,42 @@
+# airlock_RP411
+
+A community build of Red Planet 4.11, found on another site and kept here whole
+rather than cherry-picked, so the provenance of anything taken from it stays
+checkable. This is somebody else's working copy of the game: game data, the
+binaries it ran with, and the batch files that launched it.
+
+Nothing in this folder is read at build time or ships in a release. It is a
+reference, not a source. What the tree actually builds from is
+[assets/RP411/](../RP411/).
+
+## What came out of it
+
+* **`RPL4.RES`** — promoted to `assets/RP411/RPL4.RES` in `f7c7000`, after
+  checking it was a strict superset of what we had: mapping streams
+  byte-identical, `vole` identical, boots against our assets, log identical to
+  the baseline. It carries eleven vehicles and seven maps our file did not.
+* **`GAUGE/L4GAUGE.CFG`** — promoted alongside it. Adds `blktrnInit`,
+  `neutInit` and eight `blkr*Init` blocks, and fixes `dragonInit` to include
+  `twoBoosterInit`.
+* **`Console Configs/`** — copied to
+  [tools/console-config/](../../tools/console-config/). The only place the
+  vehicles, hull classes and tracks are named in words; both pages read it.
+
+## What is still only here
+
+* **`AUDIO/*.wav`** — all 223 differ from ours. Whether they are better
+  masters or just different renderings has not been established, so they have
+  not been promoted.
+* `AUDIO/LiftCut01c.wav`, `AUDIO/ReleaseMine01a.wav` and two `temp` files have
+  no counterpart in `assets/RP411/` at all.
+
+Everything else — 767 of the 998 files — is byte-identical to
+`assets/RP411/`, so git stores it once and tracking this folder costs far less
+than its 111 MB on disk suggests.
+
+## The binaries
+
+`rpl4opt.exe`, `sleep.exe` and `libsndfile-1.dll` are the community build's own,
+not ours. `rpl4opt.exe` here is a 4.11 binary and is not what
+[BUILD.md](../../BUILD.md) produces. Do not ship or run it expecting our
+behaviour; it is kept so the archive stays intact.