meta: burndown for ino preprocessing and PlatformIO compatibility audit #652

Context

The .ino and PlatformIO compatibility audit surfaced several feature gaps and hardening tasks. This parent issue tracks the burndown so the work can be implemented and closed in focused slices.

Child issues

• feat: support PlatformIO monitor_filters #644
• feat: support PlatformIO check_tool metadata #645
• feat: make Arduino.h injection conditional during ino preprocessing #646
• feat: match Arduino CLI and PlatformIO behavior for multiple ino files #647
• feat: replace regex ino prototype generation with clang AST parsing #648
• fix: ensure generated ino cpp reuse is stable on Windows paths and line endings #649
• feat: warn when main.cpp causes ino preprocessing to be skipped #650
• feat: support cxx source files in source scanner #651

Scope

This tracker covers:

• PlatformIO monitor_filters support in generated platformio.ini.
• PlatformIO check tooling metadata support, including check_tool, for pio check workflows.
• Conditional Arduino.h injection during .ino preprocessing.
• Compatibility investigation for multiple .ino file ordering/naming against Arduino CLI and PlatformIO.
• Replacing brittle regex prototype generation with clang AST or equivalent parser-backed handling.
• Stable generated .ino.cpp reuse on Windows paths and line endings.
• Yellow warning when main.cpp causes .ino preprocessing to be skipped.
• .cxx source file support in the source scanner.

Acceptance criteria

• Each child issue is resolved or intentionally closed with rationale.
• User-visible behavior is documented where applicable.
• Tests cover the compatibility and cache-stability behavior added by each child issue.
• The final implementation avoids unnecessary generated-source rewrites that would invalidate compiler caching.

Related issues

• feat: support PlatformIO monitor_filters #644
• feat: support PlatformIO check_tool metadata #645
• feat: make Arduino.h injection conditional during ino preprocessing #646
• feat: match Arduino CLI and PlatformIO behavior for multiple ino files #647
• feat: replace regex ino prototype generation with clang AST parsing #648
• fix: ensure generated ino cpp reuse is stable on Windows paths and line endings #649
• feat: warn when main.cpp causes ino preprocessing to be skipped #650
• feat: support cxx source files in source scanner #651