Share one metadata walk between Mono and IL2CPP

[ero-qt]

Part of #141.

Moves the Mono and IL2CPP metadata walks into one private managed module. On master the two walks are near copies. field.rs is the same file on both sides, the two pointer.rs files share 160 lines apart from one dereference, and class and field lookup match line for line. The real differences sit behind a Runtime enum that is matched exhaustively, so a new runtime is a compile error at every point where the runtimes differ. The public API stays the same on both sides.

Only the last two commits are new. The parity tests land first. They are hand-laid images of both runtimes' structures at real builds' offsets, and they pin behavior before any code moves.

The internal field lookup returns the declaring class, which nothing uses yet. A follow-up resolves inherited statics against the right static table, and I shaped the seam once. The field entry size is named stride, one name for Mono's alignment and IL2CPP's struct_size.

Two doc links to Image that never resolved are fixed.

cargo test --all-features runs the suite. The new tests cover image, class, field and static lookup, pointer paths, and field climbs on both runtimes.

[CryZe]

Can you rebase this pull request so we can continue reviewing?

[CryZe]

The shared managed walk preserves the public Mono and IL2CPP behavior, with the runtime-specific differences kept explicit and parity fixtures covering image, class, field, static-table, and pointer-path traversal. I also built the Lunistice autosplitter in release mode against this exact head. The controlled WASM comparison grew by 1,626 bytes (48,234 to 49,860), which is acceptable for this refactor but worth tracking as the known-version path is optimized later. All local and CI tests pass.