Let a script run a script, four deep

A build script calling a setup script is the first thing anybody tries.

What is saved when one script starts another is A POSITION AND NOT A
BUFFER: the name, which block comes next, how many are left, and where in
the block it had got to. Seventy bytes, and they sit next to each other in
the data segment on purpose so that saving them is one copy. The block
itself is read again on the way back, which costs one disk read per return
and saves 257 bytes a level - the inner script reads its own block into the
single buffer there is, so coming back means fetching the outer one's block
again and landing on the byte it left.

The slot is reached by stepping rather than by multiplying, because this
machine has no multiply and the depth is never more than three steps.

Four levels. Deep enough for a script calling a script that calls a helper,
shallow enough that a script running itself says so rather than filling
memory. A line that fails now stops every level and not just the innermost,
because a build whose helper failed should not carry on in its caller.

The caller's place is saved BEFORE the new file is looked at, and put back
on every way out that is not success. Opening writes the name into the live
state in order to ask the disk about it, so by the time "there is no such
file" is known, the caller's place has already been overwritten - a failed
'do' inside a script would otherwise leave the script that ran it reading
from a name it never chose.

The test resumes in the outer script's SECOND block, which is the case the
whole design turns on and the one an ordinary nesting test would miss.
Breaking the re-read, the save, or the limit each fails it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW

Tests/input/cosmosScriptNest.in

@@ -0,0 +1,3 @@
+do outer.script
+do loop.script
+exit