mods: let games expose loading speed as a free integer, plus disabled_by

Players asked for CD speeds beyond the curated 2x/4x list and for the two
loading accelerators to stop being one control (GH TombaRecomp#5: a
speedrunner wants shorter loads but NOT the host speed-up, which runs past
the loading screen and breaks a skip).

Three changes, all general:

- psx_mod_set_disc_speed accepted only divisor 2 or 4. That was policy,
  not safety: cdrom.c divides the sector delay and floors the result at
  CDROM_MIN_DELAY, and apply_speed leaves XA streaming at authentic
  timing, so no positive divisor can produce a zero-delay storm or
  pitch-shift FMV audio. Accept 1..PSX_MOD_DISC_SPEED_MAX and let the
  mod's description carry the risk warning.

- psx_mod_set_load_acceleration accepted 2..16. Host pacing cannot desync
  the guest at all (every frame, CD deadline, interrupt and callback still
  happens), so the ceiling was arbitrary. Accept 1..PSX_MOD_LOAD_ACCEL_MAX,
  with 1 meaning authentic and 0 still meaning uncapped.

- New option key disabled_by: an option names a BOOLEAN sibling that
  overrides it, so "tick Instant and the speed box stops mattering" needs
  no second widget type. The parser rejects a dangling, self-referential
  or non-boolean reference rather than leaving a control silently always
  enabled, and the provider resolves the link against the live selection
  so the launcher and the plugins can never disagree about whether a
  control counts.

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

runtime/include/mod_plugins.h

@@ -123,10 +123,20 @@ enum {
 int psx_mod_set_frame_interpolation_blend(uint32_t blend_mode);
 int psx_mod_set_auto_skip_fmv(int enabled);
 
+/*
+ * Upper bounds for the two loading-speed knobs below. Both are generous on
+ * purpose: games surface them to players as free-form integers, and neither
+ * can corrupt guest state (see the notes on each setter). They exist to reject
+ * nonsense, not to curate a list of "blessed" speeds.
+ */
+#define PSX_MOD_LOAD_ACCEL_MAX  1024u
+#define PSX_MOD_DISC_SPEED_MAX  1024u
+
 /*
  * Accelerate only the wall-clock pacing of sustained non-XA data loads while
  * preserving every guest VBlank, CD deadline, interrupt, and callback.
- * wall_clock_multiplier accepts 2..16, or zero for uncapped host speed.
+ * wall_clock_multiplier accepts 1..PSX_MOD_LOAD_ACCEL_MAX, or zero for
+ * uncapped host speed (1 is a no-op, i.e. authentic pacing).
  * release_frames controls how many guest frames acceleration may remain active
  * after the load predicate clears; zero is the precise/speedrun-safe policy.
  */
@@ -134,10 +144,15 @@ int psx_mod_set_load_acceleration(uint32_t wall_clock_multiplier,
                                   uint32_t release_frames);
 
 /*
- * Select guest-visible CD timing for a game-owned loading feature. divisor is
- * 2 or 4, or zero for the bounded "instant" scheduler; instant_max_per_frame
- * is used only with divisor zero. Unlike host load acceleration, this changes
- * when the emulated game receives CD interrupts and can expose timing bugs.
+ * Select guest-visible CD timing for a game-owned loading feature. divisor
+ * divides the emulated sector delay, so it IS the speed multiplier: 1 is
+ * authentic timing, higher is faster, up to PSX_MOD_DISC_SPEED_MAX. Zero
+ * selects the bounded "instant" scheduler, and instant_max_per_frame (1..256)
+ * applies only in that case. cdrom.c floors the divided delay at
+ * CDROM_MIN_DELAY and leaves XA streaming at authentic timing, so no value
+ * here can produce a zero-delay storm or speed up FMV audio. Unlike host load
+ * acceleration this changes WHEN the guest receives CD interrupts and can
+ * expose game timing bugs, which is why it is a separate, opt-in knob.
  */
 int psx_mod_set_disc_speed(uint32_t divisor,
                            uint32_t instant_max_per_frame);