Take a device's line down when its status port is read

A device raises a line and something has to take it down. Two things did:
being interrupted, and being woken from WAIT with the Interrupt Flag down -
the second because a masked program has nowhere to dispatch to, so nobody
else would.

There was a third way to learn a device had finished and nothing answered
it. The documented idiom reads the status, branches out if the device is
already done, and only WAITs otherwise; on a disk quick enough to finish
before the first look, which is every disk here, the WAIT is unreachable.
The line then stood for the rest of the machine's life.

The program that leaves it standing never pays for it - it was masked
throughout. The bill arrives at whoever next sets the Interrupt Flag. The
boot chain reads the disk to load a program, leaves the line up, and hands
over; the loaded program is then interrupted on behalf of a read that
finished before it existed, through a vector table with no entry for a
device it never touched, and faults on the instruction after its SIF.

Found by running Examples/tune.asm through Once. It set up its whole sound
and died four bytes before its first note, which is why it was silent
rather than wrong - and why it looked like a sound bug for a while.

So reading the port that answers a device takes its line down, the same way
taking the byte already took the console's down. Disk and screen do it on
their status port. And a reset now clears every line, which is the sentence
the manual already makes about the vector table: a handler left behind aims
an interrupt into a program that is no longer running, and so does a line.

testPrograms/diskLineTest.asm pins it - the racy idiom, then SIF with no
handler installed anywhere. It faults without the fix.

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

Tests/expected/diskLineTest.out

@@ -0,0 +1,3 @@
+no line
+Execution halted.
+[exit 0]

Tests/manifest

@@ -750,6 +750,13 @@ runOffTest           | testPrograms/runOffTest.asm        | run      | -
 # that never woke would hang, and a hang is what this catches. Whether it slept or spun is
 # a question about cycle counts, which settle() strips, so Tests/terminal.sh asks that one.
 waitTest             | testPrograms/waitTest.asm          | run      | -                  | 5000000 | disks/sbfs.img@10000
+# The other half of what WAIT is for, and the half that was missing. waitTest checks that a
+# masked program which SLEEPS on a device is woken and does not leave the line up; this checks
+# the program that never sleeps at all, because the disk finished before the first look. The
+# WAIT is unreachable, so nothing but the status read can take the line down. It then sets the
+# Interrupt Flag with no handler installed anywhere: a standing line faults there, which is
+# how a program loaded by CosmOS used to die on the instruction after its SIF.
+diskLineTest         | testPrograms/diskLineTest.asm      | run      | -                  | 1000000 | disks/sbfs.img
 printDecimalTest     | testPrograms/printDecimalTest.asm  | xfail    | -                  | -
 printDigitTest       | testPrograms/printDigitTest.asm    | xfail    | -                  | -
 printHexTest         | testPrograms/printHexTest.asm      | xfail    | -                  | -