phase1.0b: declare cycle-paced SIO state vars (inert, macro-guarded)

Adds cycle-paced bus model state under #if SIO_MODEL_CYCLE_PACED. Macro
still defaults to 0 (Phase 1.0a stance), so the entire block preprocesses
out — zero references in any compiled path, zero behavior change. Phase
1.0c will flip the default and add the dispatch quantum tick; Phase 1.0d
will wire these into the TX path.

Declared (none used yet):
- SIO_BAUD_CYCLES_DEFAULT (1088), SIO_ACK_CYCLES_DEFAULT (170)
- sio_tick_quantum_cycles
- TX shifter: sio_shift_active/byte/remaining
- TX one-byte buffer: sio_tx_buffered/buffer (no$psx semantics)
- ACK delay: sio_pending_ack/ack_remaining
- SioBusOwner enum + sio_bus_owner / sio_bus_byte_index for SELECT-
  lifetime ownership tracking
- Counters: tx_writes_buffered/dropped_busy/dropped_cross_device,
  cross_device_pad_during_card, tx_buffer_promoted{,_during_card},
  pad_byte_processed_in_card_data (hard-failure assertion at 1.0d),
  shift_completes, ack_fires

Validation against 5670423: identical state at t=20s (tx_writes=960,
exception_entries=1212) and t=30s (tx_writes=5048); pad polling armed
5304/5304 no_ack=0; 4x CROSS succeeds; card txn distribution unchanged
(10 long txns reach bytes=15 ts=10); txn 0 still reads card data with
RX bytes 10-11 = 0x4D 0x43 ("MC" header magic).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

runtime/src/sio.c

@@ -1196,6 +1196,61 @@ void sio_get_burst_stats(uint64_t out[10]) {
 #define SIO_MODEL_CYCLE_PACED 0
 #endif
 
+#if SIO_MODEL_CYCLE_PACED
+/* ---- Phase 1.0b: cycle-paced SIO state (inert) ---------------------------
+ *
+ * Declared under macro guard so they exist only in cycle-paced builds.
+ * Phase 1.0b adds the variables; Phase 1.0c+ wires them into sio_tick /
+ * sio_write SIO_TX_DATA / sio_write SIO_CTRL paths. With the macro
+ * defaulting to 0 (Phase 1.0a/b posture), no code references these — they
+ * are file-scope statics with no callers, no readers, no telemetry hooks.
+ *
+ * Hardware-time constants. PSX SIO0 default baud (BAUD=0x88, factor 8) is
+ * approximately 1088 CPU cycles per byte; the device-side ACK arrives
+ * about 170 cycles after the byte completes shifting on the bus. */
+#define SIO_BAUD_CYCLES_DEFAULT 1088
+#define SIO_ACK_CYCLES_DEFAULT  170
+
+/* Dispatch-loop quantum: SIO time advance per !in_exception
+ * psx_check_interrupts call. Tunable in Phase 1.0c+ via TCP. Phase 1.0b:
+ * declared only, never referenced. */
+static int sio_tick_quantum_cycles = 64;
+
+/* TX shifter and one-byte TX buffer modelling JOY_TX_DATA. */
+static int     sio_shift_active     = 0;
+static uint8_t sio_shift_byte       = 0;
+static int     sio_shift_remaining  = 0;
+static int     sio_tx_buffered      = 0;
+static uint8_t sio_tx_buffer        = 0;
+static int     sio_pending_ack      = 0;
+static int     sio_ack_remaining    = 0;
+
+/* Bus ownership over the SELECT lifetime. Set on FIRST accepted byte of
+ * each SELECT-asserted lifetime; never reclassified. Cleared on SELECT
+ * deassert / RESET / TX_EN clear. Not used in 1.0b. */
+typedef enum {
+    SIO_OWNER_NONE    = 0,
+    SIO_OWNER_CARD    = 1,
+    SIO_OWNER_PAD     = 2,
+    SIO_OWNER_UNKNOWN = 3,
+} SioBusOwner;
+static SioBusOwner sio_bus_owner      = SIO_OWNER_NONE;
+static uint32_t    sio_bus_byte_index = 0;
+
+/* Telemetry counters (Phase 1.0d will populate; Phase 1.0b just declares).
+ * pad_byte_processed_in_card_data is the hard-failure assertion: must
+ * remain 0 once cycle-paced TX path is wired. */
+static uint64_t s_pace_tx_writes_buffered;
+static uint64_t s_pace_tx_writes_dropped_busy;
+static uint64_t s_pace_tx_writes_dropped_cross_device;
+static uint64_t s_pace_cross_device_pad_during_card;
+static uint64_t s_pace_tx_buffer_promoted;
+static uint64_t s_pace_tx_buffer_promoted_during_card;
+static uint64_t s_pace_pad_byte_processed_in_card_data;
+static uint64_t s_pace_shift_completes;
+static uint64_t s_pace_ack_fires;
+#endif /* SIO_MODEL_CYCLE_PACED */
+
 void sio_tick(int cycles) {
     (void)cycles;  /* reserved for cycle-paced model (Phase 1.0c+) */
     if (sio_irq_pending && sio_irq_countdown > 0) {