runtime: fast-forward toggle shortcut (TurboToggle / fast_forward_toggle_pad) + document the implicit Select chord

[kerokline]

Follow-up to #307 (controller host shortcut for fast-forward). Two things a player hit on Breath of Fire III:

1. Turbo is hold-only, so there was no way to lock fast-forward on and put the controller down.
2. A host shortcut bound to a single trigger ("righttrigger+") never fired, because hotkey_pad_binding_down() treats any non-chord binding as an implicit Select + X and nothing told the player that.

Fast-forward toggle

• host_keymap: new HOST_KEYMAP_TURBO_TOGGLE, [KeyMap] TurboToggle, default F9.
• main.cpp: g_manual_turbo_latched, flipped on the key edge or by a fourth host shortcut, PSX_ASSIST_BIND_FAST_FORWARD_TOGGLE ("Fast-forward toggle", [hotkeys] fast_forward_toggle_pad, unbound by default), polled next to Rewind / Save states through the same hotkey_pad_binding_down() matcher. While latched the existing manual fast-forward block runs exactly as if Turbo were held, so the PSX_FAST_FORWARD_SPEED cap and presentation cadence are unchanged. OSD: Fast forward: 4x (locked) on, Fast forward: off when released. The hold shortcut's own edge message is suppressed while latched so the two never double-post.
• Threaded through every settings hop (settings.toml → globals → launcher seed → LauncherSettings → back), mirroring fast_forward_pad; config_loader reads/writes [hotkeys] fast_forward_toggle_pad under the same pad_bind_value_ok bound.
• test_rewind_toggle_combo.py guards the new sites (including that the toggle appears at every hop the hold binding does).

Implicit Select chord, documented

README now states that a controller host shortcut bound to one button or one trigger direction is a chord with Select, and that only a two-button capture replaces the implicit Select. The launcher companion (RetroPortingToolKit/recomp-ui#48) renders such bindings as select + … and adds the fourth "Fast-forward toggle" row. Without #48 this PR is still complete: the extra assist_binding_labels entry is simply drawn by the launcher's generic host-shortcut loop, and the keyboard F9 path needs no launcher support.

Verified (BoF3, RelWithDebInfo)

• Launcher shows the fourth row; a seeded fast_forward_toggle_pad = 109 survives the launcher round-trip and is written back to settings.toml.
• In-game F9 latches (OSD … (locked)), second F9 shows Fast forward: off.
• python runtime/tests/test_rewind_toggle_combo.py passes.

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