virtualbox: remove the FDP access method

[Wenzel]

Closes #289.

Breaking clean cutover:

• deletes the FDP VirtualBox driver and fdp/fdp-sys dependencies;
• removes every icebox/libFDP clone/build/include/library step from CI and Python/Gitpod containers;
• virtualbox now aggregates virtualbox-api + virtualbox-vmi; Linux CI covers each standalone method and aggregate cfg, Windows CI compiles virtualbox-api only;
• removes FDP from Rust/Python/C parameters, exports, examples, Volatility parsing and docs. VirtualBox access-method selection is explicit; Rust returns AccessMethodRequired; C keeps a validated u32 discriminant (0 = no VBox params, unknown values rejected without enum UB);
• replaces legacy docs with MachineDebugger/VBoxVMI docs; adds ADR 0003-drop-fdp-access-method.md and CHANGELOG breaking-removal entry.

Platform note: both replacement methods currently work on Linux. Windows virtualbox-api is compile-only because rs-vboxapi loads VBoxXPCOMC.so, not VBoxCAPI.dll; after FDP removal no VirtualBox runtime method works on Windows. Docs/artifact guide state this explicitly.

Verification: Rust 1.99 fmt/clippy default/api/vmi/aggregate/aggregate-tests; lib/error/filter/utilities tests; Linux all-target aggregate check; Windows GNU api-only cross-check; Python 3.14 compile/unit/build/import; C header generation + 4 examples; CI YAML, Dockerfiles, mdbook. Cargo.lock/product/CI/container grep contains no FDP/icebox; only CHANGELOG and historical ADRs retain intentional mentions.