Breakpoints: SWI osBreak, and s refuses a read only bank

A breakpoint that shows every register as the program had them, waits for a
key, and carries on.

NOTHING IS OVERWRITTEN, and that is the design rather than a shortcut. A
breakpoint poked into a running program has to replace an instruction, and
putting that instruction back in order to continue is the same act as
disarming the breakpoint; firing a second time would mean stepping over the
restored instruction and putting the breakpoint back behind it, and this
machine cannot step a single instruction. SWI is two bytes, dispatches
through a vector, and its frame already holds the address after it, so RETI
resumes at the next instruction with nothing to restore and nothing to
re-arm. It fires every time it is reached.

The price is that a breakpoint is part of the program: a build with them in
has different addresses from a build without. That is the bargain every
machine with a break instruction makes.

Every value shown comes out of the frame rather than the registers, because
by the time the handler runs the registers are the handler's. Apps/Break.asm
stops twice so that the second stop is checked as well as the first.

Also here, found by the test that came with it: the monitor's s wrote into
whichever bank was selected, and bank 2 is the controller's own table,
published read only. Writing to it was refused, and a refusal nobody catches
stops the machine - so selecting the bank table to look at it and then typing
s killed the session. bankPresent now keeps the whole flags byte and s
declines. The recorded output of cosmosMonitor had contained that crash,
having been blessed without being read.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Tests/manifest

@@ -221,6 +221,10 @@ cosmosRun            | CosmOS/Source/cosmos.asm           | run      | cosmosRun
 # The last part is what the mode is FOR: a program typed in as bytes, run with g, and the
 # prompt that comes back is the monitor's own. A program giving the machine back lands where
 # it was started from, so looking at something and running it do not interrupt each other.
+#
+# It also asks to write into bank 2, the controller's own table, which is published read
+# only. That used to stop the machine, and the recorded output of this test contained the
+# crash without anybody noticing, which is what blessing a result without reading it buys.
 cosmosMonitor        | CosmOS/Source/cosmos.asm           | run      | cosmosMonitor.in   | -     | disks/cosmos.img
 # The original hello.asm, brought over as an application. It is not much of a program,
 # but it is the one that talks to the hardware directly: it writes to port 0x00 instead
@@ -270,6 +274,13 @@ cosmosFiles          | CosmOS/Source/cosmos.asm           | run      | cosmosFil
 # name, and with several words, since what arrives is the rest of the line rather than a
 # list and it is the program's business what to make of it.
 cosmosSay            | CosmOS/Source/cosmos.asm           | run      | cosmosSay.in       | -     | disks/cosmos.img
+# A program stopping itself to be looked at. Two breakpoints, so what is checked is not only
+# that one fires but that the SECOND one does - which is the whole difference between this
+# design and one that overwrites an instruction, since an overwritten instruction has to be
+# put back to continue and putting it back is the same act as disarming the breakpoint.
+# Every value shown comes out of the interrupt frame rather than the registers, because by
+# the time the handler runs the registers are the handler's.
+cosmosBreak          | CosmOS/Source/cosmos.asm           | run      | cosmosBreak.in     | -     | disks/cosmos.img
 # The editor, which is the first program on this machine that makes a file a person typed:
 # every byte on every other image here was put there by the host tool. It is run twice in
 # one session, and that is the test rather than a flourish - the second run reads back what
@@ -291,6 +302,7 @@ app-Life             | CosmOS/Apps/Life.asm               | assemble | -
 app-Snake            | CosmOS/Apps/Snake.asm              | assemble | -                  | -
 app-Keys             | CosmOS/Apps/Keys.asm               | assemble | -                  | -
 app-Say              | CosmOS/Apps/Say.asm                | assemble | -                  | -
+app-Break            | CosmOS/Apps/Break.asm              | assemble | -                  | -
 app-Edit             | CosmOS/Apps/Edit.asm               | assemble | -                  | -
 app-Files            | CosmOS/Apps/Files.asm              | assemble | -                  | -