Fix VIA port B data direction register handling for joystick fire buttons

- Correctly handle DDR in recalculatePortBPins(): output pins use ORB values, input pins default high
- Fix inverted button logic: clear pin bits when buttons are pressed (active low)
- Remove redundant DDR masking that was preventing proper input pin reads
- This fixes ADVAL(0) not detecting mouse joystick fire button presses

The original code wasn't respecting the Data Direction Register, causing joystick
fire button states to not be readable by BBC BASIC's ADVAL(0) function.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: mattgodbolt

src/via.js

@@ -369,7 +369,7 @@ class Via {
     portBUpdated() {}
 
     getJoysticks() {
-        return { button1: true, button2: true };
+        return { button1: false, button2: false };
     }
 
     recalculatePortAPins() {
@@ -381,18 +381,16 @@ class Via {
 
     recalculatePortBPins() {
         const prevPb6 = !!(this.portbpins & 0x40);
-        this.portbpins = this.orb & this.ddrb;
-        const buttons = this.getJoysticks();
+        // Start with output pins, and inputs all high
+        this.portbpins = (this.orb & this.ddrb) | (~this.ddrb & 0xff);
 
         // AUG p418
         // ### PB4 and PB5 inputs
-        // These are the inputs from the joystick FIRE buttons. They are
-        // normally at logic 1 with no button pressed and change to 0 when a
-        // button is pressed
-        if (!buttons.button1) this.portbpins |= 1 << 4;
-        if (!buttons.button2) this.portbpins |= 1 << 5;
+        // These are the inputs from the joystick FIRE buttons. They are active low.
+        const buttons = this.getJoysticks();
+        if (buttons.button1) this.portbpins &= ~(1 << 4); // Clear PB4 if button1 pressed
+        if (buttons.button2) this.portbpins &= ~(1 << 5); // Clear PB5 if button2 pressed
 
-        this.portbpins |= ~this.ddrb & 0xff;
         this.drivePortB();
         if (prevPb6 && !(this.portbpins & 0x40)) {
             // If we see a high to low transition on pb6, and we are in timer2 pulse counting mode, count a pulse.