feat: add AD9629 register definitions

[IM-TechieScientist]

Adds the initial AD9629 platform driver files for #201. PR is dependent on #206.

• Adds src/platform/ad9629.h and src/platform/ad9629.c.
• Defines AD9629 register addresses from the datasheet memory map.
• Adds bit masks for SPI config, modes, clock divide, test mode, BIST, output mode, output adjust, output phase, output delay, OR/MODE select and USR2.
• Adds typed enums for speed grade, power mode, clock divide, output format, output drive, DCO polarity, test modes, user pattern modes and output delay steps.
• Adds the public AD9629 control API scaffold for register access, reset/probe, output configuration, test patterns, BIST and lowrate GCLK/SDIO controls.

Summary by Sourcery

Introduce an ADC frontend abstraction layer and scaffold AD9629 external ADC support while migrating DSO and mixed-signal instruments to the new interface.

New Features:

• Add a generic ADC frontend interface with backend selection, capability querying, and capture orchestration.
• Add an internal ADC frontend backend that wraps the existing adc_capture implementation.
• Add AD9629 platform driver and public control API with register definitions and typed configuration/controls for the external parallel ADC.

Enhancements:

• Update DSO and mixed-signal instruments to use the ADC frontend interface and backend capabilities instead of direct adc_capture constants.

Build:

• Wire new ADC frontend and AD9629 driver sources into the CMake build and register the internal ADC frontend during platform initialization.

[sourcery-ai]

Reviewer's Guide

Introduces a generic ADC frontend abstraction with an internal backend driver, migrates DSO and mixed-signal instruments to this interface, and scaffolds a new AD9629 external ADC register/control driver with typed configuration and API but stubbed hardware operations.

File-Level Changes

Change	Details	Files
Add AD9629 external ADC register map, configuration types, and control API with stubbed implementations.	Define AD9629 register addresses, bit masks, enums, and configuration/handle types for SPI control and output/test features. Implement default configuration, init/deinit, and placeholder functions for all AD9629 register access and control operations that currently return false. Document that commit only scaffolds the control plane and leaves actual SPI transactions for later work.	src/platform/ad9629.h src/platform/ad9629.c
Introduce an ADC frontend abstraction layer and register an internal backend that adapts the existing adc_capture implementation.	Define AdcFrontendBackend, capabilities, config, capture info, driver vtable, and frontend API for registering drivers, selecting backends, and performing captures. Implement adc_frontend.c with driver registry, active-backend selection, capture orchestration (run/arm/start/wait/abort), capability queries, and a completion callback mechanism. Add internal_adc_frontend adapter that wraps adc_capture into an AdcFrontendDriver, mapping configs and capture info and exposing capabilities and operations. Register the internal ADC frontend driver during platform initialization and wire new sources into the build system.	src/system/instrument/adc_frontend.h src/system/instrument/adc_frontend.c src/platform/internal_adc_frontend.h src/platform/internal_adc_frontend.c src/platform/platform.c CMakeLists.txt
Refactor DSO and mixed-signal instrument code to use the new ADC frontend interface instead of adc_capture directly.	Replace adc_capture includes with adc_frontend where appropriate and update comments to reference the ADC frontend interface. Use adc_frontend_get_capabilities to validate channels and sample rates dynamically rather than static ADC_CAPTURE_* constants. Update configuration and capture flows in dso and mixed_signal modules to use AdcFrontendConfig, AdcFrontendCaptureInfo, and adc_frontend_* functions for init/configure/run/arm/start/wait/read_once/is_busy and GPIO mapping. Add small helper functions for reusing capability queries and remove redundant "!initialized" checks now handled by adc_frontend_configure/apply_config.	src/system/instrument/dso.c src/system/instrument/mixed_signal.c

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