Add ForeignPointer for C/FFI handle ownership

[Copilot]

Extends safety framework with ForeignPointer<T, Deleter> to replace std::unique_ptr for C/FFI/GPU/OS handles, keeping all ownership abstractions within the framework.

Implementation

• safety/foreign_pointer.h: RAII wrapper with custom deleter support, no-throw semantics, move-only. Includes self-assignment protection in reset() to prevent double-free (safer than std::unique_ptr's UB).

• .clang-tidy: Added safety-foreignpointer-for-c-api enforcement rule alongside existing safety-smartpointer-in-try.

• README.md: Updated ownership model to 4 patterns. Documents that ForeignPointer is required for foreign handles and forbids std::unique_ptr/SafePointer/SmartPointer for C APIs.

• main.cpp: Demonstrates proper usage with FILE* and C API contexts, including NULL checking pattern.

Usage

struct FileDeleter {
    void operator()(FILE* f) const noexcept {
        if (f) fclose(f);
    }
};

FILE* raw = fopen("data.txt", "r");
if (raw) {
    safety::ForeignPointer<FILE, FileDeleter> file(raw);
    // Automatic cleanup on scope exit
}

Ownership Rules

• Application objects: SafePointer, SafeResultPointer
• Startup/throwing: SmartPointer (inside try/catch only)
• C/FFI/GPU/OS handles: ForeignPointer (this PR)
• Forbidden: std::unique_ptr for foreign handles, raw owning pointers

Original prompt

Add a clang tidy rule and implement following, update readme accordingly.
IMPORTANT SAFETY FRAMEWORK EXTENSION – DO NOT DEVIATE

We are extending our existing safety framework with a new ownership type
called safety::ForeignPointer.

GOAL:

Replace std::unique_ptr usage for owned C / FFI handles
Keep all ownership inside our safety framework
Do NOT use SafePointer or SmartPointer for C API handles
RULES:

Do NOT introduce raw owning pointers (T*).
Application-level ownership must use:
safety::SafePointer
safety::SafeResultPointer
Throwing ownership is ONLY allowed via:
safety::SmartPointer (inside try/catch at startup boundaries)
C / FFI / GPU / OS handles MUST use:
safety::ForeignPointer<T, Deleter>
Do NOT use std::unique_ptr for C API ownership.
Do NOT add safety::SafeBoundary except at true entry points.
Worker threads MUST begin with safety::SafeScope.
No exceptions may escape worker threads.
IMPLEMENTATION REQUIRED
Create a new file:
safety/foreign_pointer.h

WITH THIS EXACT IMPLEMENTATION:

#pragma once
#include

namespace safety {

template <typename T, typename Deleter>
class ForeignPointer {
public:
ForeignPointer() noexcept : ptr_(nullptr), deleter_() {}

explicit ForeignPointer(T* ptr, Deleter deleter = Deleter{}) noexcept
    : ptr_(ptr), deleter_(deleter) {}

~ForeignPointer() noexcept {
    reset();
}

ForeignPointer(const ForeignPointer&) = delete;
ForeignPointer& operator=(const ForeignPointer&) = delete;

ForeignPointer(ForeignPointer&& other) noexcept
    : ptr_(other.ptr_), deleter_(std::move(other.deleter_)) {
    other.ptr_ = nullptr;
}

ForeignPointer& operator=(ForeignPointer&& other) noexcept {
    if (this != &other) {
        reset();
        ptr_ = other.ptr_;
        deleter_ = std::move(other.deleter_);
        other.ptr_ = nullptr;
    }
    return *this;
}

T* get() const noexcept { return ptr_; }
explicit operator bool() const noexcept { return ptr_ != nullptr; }

void reset(T* ptr = nullptr) noexcept {
    if (ptr_) {
        deleter_(ptr_);
    }
    ptr_ = ptr;
}

private:
T* ptr_;
Deleter deleter_;
};

} // namespace safety
REFactor REQUIRED
Refactor STTService to:

Remove std::unique_ptr<whisper_context>

Use safety::ForeignPointer<whisper_context, WhisperDeleter>

Example:

cpp
Copy code
struct WhisperDeleter {
void operator()(whisper_context* ctx) const noexcept {
whisper_free(ctx);
}
};

safety::ForeignPointer<whisper_context, WhisperDeleter> ctx_;
Replace all ctx_.get() usage accordingly.

DOCUMENTATION
Update README.md to include:

ForeignPointer is REQUIRED for owned C / FFI / GPU / OS handles

SafePointer is for application objects only

SmartPointer is startup-only and throwing

Raw owning pointers are forbidden

CONSTRAINTS
Do NOT introduce new pointer abstractions

Do NOT change existing SafePointer semantics

Do NOT weaken exception safety

All code must compile with clang++ 19

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