Play reads a tune from a file

The loader, and the reason it is small: everything in a tune is an offset
from wherever it was put, so taking one means adding the base to two
tables and pointing four voices at their order lists. No sequence is
walked. Nothing inside one is an address to be found and corrected, which
is the difference between a malformed tune that plays wrongly and one
that takes the loader with it.

  "SBTU", version, the tick in cycles, how many patches and sequences,
  offsets to the two tables, an order list each, and the patch each
  voice starts on.

The magic is checked before anything else, because from there on the
loader follows what the offsets name. break.sh shows what that guard is
worth: without it, handing Play a PROGRAM runs the machine away until the
cycle limit, rather than saying it is not a tune.

PatchTable, SequenceTable and VoiceStart became pointers, so the engine
does not care whether a tune came out of a file or was assembled in.
Play's built-in tune now hands over the same three addresses a loaded one
would, in eight lines - which is what keeps the two paths from drifting,
and what made this rung change no scheduler code at all.

Tests/maketune.py lays the fixture out byte by byte. IT IS NOT THE
COMPILER: the sequences and the patches are literal bytes and the only
thing computed is where each piece lands. That is the point - the loader
is checked by something that does not share its idea of the format, which
is the same reason SplitDisk and sbfs.asm share nothing but a
specification.

Both notes in the fixture are number 60, so the octave between them is a
0x80 command loading the second patch out of the file: the header, the
tick, the relocation, an order list and a patch from a file, measured in
one go. Play and the tune live on quiet.img rather than cosmos.img, so a
fixture does not move ten recordings every time it changes size.

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

Tests/expected/cosmosCrossDisk.out

@@ -32,7 +32,7 @@ vars.script             50
 blocks.script           343
 loops.script            272
 tune.sbx                318
-Play.sbx                939
+Play.sbx                2525
 notes.txt               21
 Apps                    <dir>
 hi.script               121