Platform-agnostic neutral-pin mechanism (set_input_pinned/pin_value) on
ModeBase, lifted from ebb953a. Consumed by the firmware Joystick Dual/Single
settings menu; the browser fixed-MLP<4> half of ebb953a is superseded by main's
MLP<32> N-D input work and intentionally dropped.
Adds two foundational pieces shared by every concrete mode:
- `base.hpp`: defines `nisps::ParamSchema` (the aggregate type the
`Mode` concept's `param_schema()` returns by const-reference),
`nisps::ControlEvent`, and `ModeBase<Derived, EngineT, MLPType,
NInputs>` — a CRTP scaffold absorbing input forwarding, ML
inference + voice space mapping, engine.set_params() routing, and
a per-mode ControlEvent ring buffer for MIDI/I2C events.
`ModeRoutesOutputsToEngine<Derived>` lets a derived mode (e.g.
SoundAnalysisMIDI) opt out of routing ML outputs to engine
params when its outputs become MIDI CCs instead.
- `voice_space.hpp`: `VoiceSpaceEntry` + `make_voice_space_entries()`
+ `find_voice_space()` helpers. Voice-space *mapping code* lives
inside engines (mirroring firmware); mode layer just selects
which voice space the engine uses by index/name.
No heap, no virtuals, no Pico/Arduino dependencies; satisfies stream
4 of architecture.md.
