Initialize oscillator history and AD envelope trigger state

[dajiaohuang]

Description

Initialize the processing history that Oscillator::Init and AdEnv::Init currently leave untouched.

• Oscillator::Process reads last_out_ for WAVE_POLYBLEP_TRI, but Init never assigns it. Automatic-storage oscillators can therefore read indeterminate history. Reinitializing an oscillator that has already processed a triangle also retains its previous integrator output, despite resetting its phase. Initialize that history to zero; phase-only Reset behavior is unchanged.
• AdEnv::Process reads trigger_ and prev_segment_ before assigning them. Initialize both so that the documented idle initial state does not depend on prior storage or a pending trigger. Fixes AdEnv does not initialize the "trigger_" data member, causing the envelope to trigger on the first call to Process () #212.

Only the missing state assignments change; parameters, waveform algorithms, and public interfaces are unchanged.

Testing

• Built the full library with CMake, GCC 13.3.0, and C++14 on Ubuntu 24.04 (WSL).
• Added a standalone host regression and reproduction commands under tests/initialization. It uses ordinary public calls and static initial storage, without depending on arbitrary uninitialized-memory contents. At 44.1, 48, and 96 kHz it processes a triangle, calls Init again, and checks that the first sample is zero, as follows from the phase-zero polyBLEP input and zero integrator history. It also calls Trigger, reinitializes the AD envelope, and verifies idle output without another trigger.
• On base 2c72eaf, the regression fails both cases at all three rates: the oscillator retains nonzero output, and the envelope enters attack with output 0.0001. With this change it passes. The fixed regression also passes AddressSanitizer and UndefinedBehaviorSanitizer.
• Additional bounded offline checks pass at all three sample rates: oscillator waveforms, OnePole sample/block equivalence, in-range Svf modulation, ADSR gate stages at sample and block control rates, and a triggered linear AD envelope.
• Ran util/fix_style.sh; retained only relevant changes. Local formatter is the Visual Studio bundled version, not CI's pinned clang-format 10.

No audio devices, playback, firmware flashing, or benchmarks were used. ARM cross-build and physical Daisy hardware acceptance were not run locally; these host checks do not substitute for them. No native Windows default-build fix is included (tracked separately in #197).

Demos / Example Code

tests/initialization/tst_initialization.cpp is an offline public-API example and regression; it does not require libDaisy.

Suggested label: bug (I do not have repository label-assignment permissions).