feat(emu): joystick, paddle & pushbutton emulation ($C061/$C062/$C070/PREAD)

[relmer]

Summary

Casso emulates no joystick, paddles, or pushbuttons. Every relevant soft
switch falls through to default and returns 0, so games that need analog
input or the Open/Closed Apple buttons cannot be controlled — even if they
otherwise boot and run.

Surfaced while validating Choplifter (#69): the title boots, the game starts,
but nothing responds. Root cause is not the disk path — it's that the
"press button to start" / fire action is gated on button 0 ($C061),
which is hardwired to always-unpressed.

Missing hardware

Register	Function	Current behavior
$C061 R	Open Apple / Button 0 (bit 7)	returns 0 (always unpressed)
$C062 R	Closed Apple / Button 1 (bit 7)	returns 0 (always unpressed)
$C063 R	Shift-key mod (bit 7)	unimplemented
$C064–$C067 R	Paddle 0–3 position (PREAD countdown)	return 0 (center)
$C070 R/W	Paddle timer strobe	no-op
$C058–$C05D	Annunciators AN0–AN3 (AN0/AN1 = joystick control lines)	not tracked

Known code facts (from docs/iie-audit.md)

• [CRITICAL] C3 — $C061/$C062 are unreachable via the bus.
  AppleIIeKeyboard::GetEnd() returns $C01F, so the button-read logic at
  AppleIIeKeyboard.cpp:36-45 is dead code. The bus dispatches
  $C061/$C062 to AppleIIeSoftSwitchBank (its $C050–$C07F range),
  which returns 0. (AppleIIeKeyboard.h:18, AppleIIeKeyboard.cpp:36)
• AppleIIeSoftSwitchBank::GetEnd() returns $C07F but $C060–$C07D all
  return 0 — paddles, tape, buttons absent. (iie-audit.md:182)
• [MINOR] N3 — Annunciators AN0–AN3 ($C058–$C05D) not tracked;
  AN0/AN1 drive the joystick port control lines. (AppleSoftSwitchBank.cpp:65)
• $C063 Shift-key read unimplemented. (iie-audit.md:340,359)

Proposed staging

1. Buttons-only (fixes audit C3, smallest, highest leverage).
   Make $C061/$C062 reachable and map button 0/1 to host keys
   (e.g. Left/Right Alt = Open/Closed Apple). Likely unblocks Choplifter's
   start + fire alongside its keyboard movement (A/D/W/S/Space/1-2-3).
   Either override AppleIIeKeyboard::GetEnd() to cover $C061-$C063
   and wire the existing dead code, or implement the reads in
   AppleIIeSoftSwitchBank. Add $C063 Shift read while there.
2. Analog paddle/joystick. Implement the PREAD timing model:
   $C070 strobe starts the RC countdown; $C064-$C067 read bit 7 = 1
   until each axis's timer (proportional to position, ~0-3.3 ms) expires.
   Map axes to a host gamepad and/or keyboard. Track AN0/AN1.

Acceptance criteria

• A game polling $C061 (e.g. Choplifter) sees the button as pressed when the
  mapped host input is active.
• PREAD ($C070 strobe + $C064-$C067 poll) returns a position
  proportional to the mapped axis, matching the //e RC timing within tolerance.
• DOS 3.3 / ProDOS boot and existing soft-switch tests stay green.

Notes

Separate feature from the Disk II copy-protection work (#67). Deferred — capturing
as backlog per discussion; not yet scheduled.