Source: Implement FileId and Source struct

[pdaxt]

Component

Foundation / Source

Task

Implement FileId and Source structs for source file management.

Acceptance Criteria

• FileId newtype wrapper (u32)
• Source struct with: id: FileId, name: String, content: String
• Source::new(name, content) -> Source
• Source::slice(span) -> &str
• Line offset cache for efficient lookups

Verify

let src = Source::new("test.nova", "let x = 42");
assert_eq!(src.slice(Span::new(4, 5)), "x");

Security Testing (REQUIRED)

• Test with empty source files
• Test with very large files (>4GB would overflow u32)
• Test Unicode edge cases (multi-byte chars, emoji)
• Test invalid Span ranges (out of bounds)
• Verify no panics on malformed input

Ref: FOUNDATION.md