feat: add adc frontend interface

[IM-TechieScientist]

Working on #201.
This PR adds a hardware agnostic ADC frontend interface for the DSO/MSO capture paths.

• Adds a generic ADC frontend registry and driver interface in system/instrument.
• Adds the current RP2350 internal ADC as the first registered backend.
• Refactors DSO and mixed signal capture to use the frontend interface instead of directly depending on platform/adc_capture.
• Keeps the existing internal ADC behavior working while creating a clean path for the future external parallel ADC backend.
• Adds frontend capabilities for channel/rate/sample count limits and sample width.

Summary by Sourcery

Introduce a hardware-agnostic ADC frontend layer and migrate existing capture paths to use it.

New Features:

• Add a generic ADC frontend abstraction with backend selection, capability querying, and capture control APIs.
• Register the RP2350 internal ADC as an ADC frontend backend with defined capabilities and capture metadata reporting.
• Add an optional capture-complete callback mechanism for ADC frontend users.

Enhancements:

• Refactor DSO and mixed-signal instruments to use the ADC frontend interface instead of the platform-specific ADC capture API while preserving behavior.
• Expose ADC capabilities such as channel limits, sample rate bounds, maximum sample count, and sample width to instrument code for validation.
• Integrate ADC frontend and internal ADC backend initialization into the platform startup sequence.

Build:

• Include the new ADC frontend core and internal ADC backend source files in the build configuration.

[sourcery-ai]

Reviewer's Guide

Introduces a hardware-agnostic ADC frontend layer and registry, wires the existing RP2350 internal ADC through this abstraction, and refactors DSO and mixed-signal capture paths to use the new interface and capability reporting instead of depending directly on platform/adc_capture.

Sequence diagram for DSO capture via ADC frontend

sequenceDiagram
    participant Dso as DsoInstrument
    participant Frontend as AdcFrontend
    participant Driver as AdcFrontendDriver_internal
    participant Capture as AdcCapture

    Dso->>Frontend: adc_frontend_init(config{backend=INTERNAL})
    Frontend->>Frontend: adc_frontend_select_backend(INTERNAL)
    Frontend->>Driver: init(config)
    Driver-->>Frontend: bool
    Frontend-->>Dso: bool

    Dso->>Frontend: adc_frontend_run(buffer, sample_count, info)
    Frontend->>Driver: run(buffer, sample_count, info)
    Driver->>Capture: adc_capture_run(buffer, sample_count, capture_info)
    Capture-->>Driver: bool
    Driver-->>Frontend: bool
    Frontend-->>Dso: bool

Loading

File-Level Changes

Change	Details	Files
Add a generic ADC frontend abstraction and driver registry for multiple ADC backends.	Introduce adc_frontend.h defining backend enums, capability/config/capture metadata structs, driver vtable, and public API for registering/selecting backends and performing captures. Implement adc_frontend.c with a small fixed registry of drivers, active-backend tracking, init/configure/deinit logic, validation of sample counts, conversion of driver results into last_info, and an optional completion callback. Expose helpers to query backend capabilities, map channels to GPIOs, and query busy state, ensuring the active backend is deinitialized when switching.	src/system/instrument/adc_frontend.h src/system/instrument/adc_frontend.c
Wrap the existing platform adc_capture implementation as the first ADC frontend backend for the RP2350 internal ADC.	Add an internal ADC frontend driver that adapts AdcFrontendConfig and AdcFrontendCaptureInfo to and from the existing AdcCaptureConfig/AdcCaptureInfo structures and API. Define driver capabilities using ADC_CAPTURE_* constants and a fixed INTERNAL_ADC_MAX_SAMPLE_COUNT and sample_width_bits=12. Provide an internal_adc_frontend_register() function and call it from PLATFORM_init() so the driver is registered at startup, and wire the new source file into the build.	src/platform/internal_adc_frontend.h src/platform/internal_adc_frontend.c src/platform/platform.c CMakeLists.txt
Refactor DSO capture path to use the ADC frontend interface and capabilities instead of platform adc_capture directly.	Replace adc_capture.h usage with adc_frontend.h, selecting ADC_FRONTEND_BACKEND_INTERNAL in configuration and capture calls. Use adc_frontend_get_capabilities() to validate requested channel and sample rate against backend limits instead of ADC_CAPTURE_* macros. Update capture initiation, streaming, trigger sampling, status, and GPIO mapping paths to call adc_frontend_init/configure/run/read_once/is_busy/channel_to_gpio and to use AdcFrontendCaptureInfo.	src/system/instrument/dso.c
Refactor mixed-signal capture to use the ADC frontend abstraction for analog captures while preserving logic analyser handling.	Replace adc_capture integration with adc_frontend calls, configuring ADC_FRONTEND_BACKEND_INTERNAL for the analog channel while leaving logic_analyser usage unchanged. Use adc_frontend_get_capabilities() to validate analog channel and sample rate and to determine max channel and rate limits. Update initiation flow to arm/start/wait/abort analog captures through adc_frontend_* functions and to consume AdcFrontendCaptureInfo instead of AdcCaptureInfo.	src/system/instrument/mixed_signal.c

Possibly linked issues

• PSLab-mini analog frontend firmware ahead of hardware availability #201: PR wires DSO/MSO to a new ADC frontend using the RP2350 internal ADC, matching the oscilloscope placeholder goal.

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