A demo recorder: --record-pad writes what --pad reads

One byte a frame, in exactly the format the player takes, so a recording
needs no conversion and there is no second format to keep in step. That
symmetry is the feature, and it makes the strongest form of the claim
testable: a recording is made OF a playback, and the bytes coming out have
to be the bytes that went in.

It exists because some inputs cannot sensibly be written by hand. Flying a
lander from one base to another is a few hundred frames of steering that
has to arrive somewhere eight cells wide, and several attempts at
authoring one got within two columns and no closer. That is a piloting
exercise rather than a test. Playing it once and keeping what happened is
the answer.

A BYTE FOR EVERY FRAME, written inside the loop that advances the
recordings rather than after it, so a machine that jumped several frames
at once still writes one for each. A recording is a timeline: one that
skipped the frames nobody looked at would play back faster than it was
flown.

What is recorded is what the DEVICE WOULD REPORT, not the live state - a
recording of a playback that wrote the live state would be a file of
noughts. And it is flushed as it goes, because a recording is usually
stopped by whoever is playing rather than by the program ending, and a
demo lost to a buffer is a demo flown twice.

Tests/replay.sh is where this and whatever follows it are checked. Twelve
scripts now.

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

makefile

@@ -358,6 +358,8 @@ test: all strict
 	@echo
 	@./Tests/agree.sh
 	@echo
+	@./Tests/replay.sh
+	@echo
 	@./Tests/docs.sh
 
 # Rebuild all three tools with the address and undefined behaviour sanitizers and run
@@ -405,6 +407,8 @@ sanitize:
 	@echo
 	@./Tests/agree.sh
 	@echo
+	@./Tests/replay.sh
+	@echo
 	@./Tests/docs.sh
 	@$(MAKE) --no-print-directory clean
 	@$(MAKE) --no-print-directory