Add runnable examples for path-based and descriptor-based file I/O

[LunaStev]

Tracked by #375.

Problem

Current file APIs are easy to misuse. fs_read_all accepts a path, destination pointer, and capacity, while io_read accepts a file descriptor. Existing examples do not make that boundary obvious.

Scope

Add small runnable examples and Wave corpus fixtures for:

• Opening and closing a file with std::fs::file.
• Reading a path into a caller-owned byte array with fs_read_all.
• Reading an already-open descriptor with std::io::fd::io_read.
• Handling negative return values.
• Reporting the number of bytes read without treating it as a string length.

Use only current APIs. API renaming belongs to #375 and its design follow-ups.

Completion criteria

• Every example compiles with the current compiler and installed std layout.
• At least one fixture executes on supported hosted Linux CI.
• Buffer capacity and returned byte count are checked.
• Examples do not import private transitive symbols.
• Documentation states that fs_read_all does not allocate or return str.

[Saket7002]

I'd like to work on this issue. I'll first look through the existing file I/O examples and add the requested runnable examples and fixtures while keeping the current APIs unchanged.

[LunaStev]

I'd like to work on this issue. I'll first look through the existing file I/O examples and add the requested runnable examples and fixtures while keeping the current APIs unchanged.

Thanks! I've assigned this issue to you.