Add Copy and Compare, which stream in both directions at once

Copy joins the read and write streaming services: source and destination are
both larger than Data Memory while the program owns one block. Compare reads
two files through separate blocks and ignores the bytes past a short final
block, which belong to neither file. Between them they exercise empty,
exact-block, part-block and 84,000 byte files, and the host extracts the copy
afterwards so that two native programs agreeing with each other is not the only
oracle.

Written by ChatGPT, as their headers record, along with the agree.sh section
and the manifest entry that drive them.

THIS SHOULD HAVE COME FIRST. The commit before it staged whole files rather
than the hunks it meant, so its manifest already names these two programs while
their source was still untracked - that commit will not build on its own. Left
in place rather than rewritten, since the pair is right and only their order is
wrong.

NOTES.md is their review of the streaming work. The first item in it is fixed
by the commit before this one; the rest are still open.

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

Programs/CosmOS/README.md

@@ -327,6 +327,8 @@ from every assembly file in it. Several are old programs written for the bare ma
 | Stream | Reads an 84,000 byte file through a buffer of 256, which is what says a file bigger than Data Memory can be read at all. |
 | Type | Prints a named text file a block at a time, including one too large to fit in Data Memory. |
 | Pour | Writes a file a block at a time, never holding more than one block of it. Each block is filled with a byte naming itself, so a block written to the wrong place shows up as content rather than as a length. |
+| Copy | Copies one path to another a block at a time, including an empty file or one larger than Data Memory. |
+| Compare | Compares two files a block at a time, stopping at their real tails rather than comparing unused bytes in the final disk blocks. |
 | Wander | Goes to the directory it is given and reads a file there by a bare name. The only thing that moves the machine from inside a program, and so the only thing that can check the shell puts the working directory back afterwards. |
 | More | A forward-only pager. Space advances a screen, Return one line, and q stops. |