feat(forge): add target attestation safety policy

[cwilson613]

Summary

Adds an explicit target-attestation safety policy for destructive forge USB operations.

This supports external-first install flows where the operator must attest the target class before destructive writes, e.g. an external USB/Thunderbolt SSD is allowed while an internal Apple NVMe target is forbidden by default.

New safety fields:

requires_target_attestation
 target_attestation
allowed_targets
forbidden_targets

Supported attestations:

external-usb-ssd
external-thunderbolt-ssd
internal-apple-nvme

Behavior

For destructive USB install requests:

• if requires_target_attestation = true and no attestation is present, planning is blocked with TARGET_ATTESTATION_REQUIRED
• if an attestation is not listed in allowed_targets, planning is blocked with TARGET_ATTESTATION_NOT_ALLOWED
• if an attestation is listed in forbidden_targets, planning is blocked with TARGET_ATTESTATION_FORBIDDEN
• internal-apple-nvme is additionally blocked by default unless allow_internal_disk_selection = true

Why

This is the next safety primitive needed for external-first NixOS install flows on hardware like Intel Apple T2 MacBooks. It lets profile/materialization artifacts express:

allowed_targets = external-usb-ssd, external-thunderbolt-ssd
forbidden_targets = internal-apple-nvme
requires_target_attestation = true

without relying only on stringly disk names.

Tests

nix shell nixpkgs#cargo nixpkgs#rustc nixpkgs#rustfmt -c cargo fmt
nix shell nixpkgs#cargo nixpkgs#rustc -c cargo test target_attestation -- --nocapture
nix shell nixpkgs#cargo nixpkgs#rustc -c cargo test

Targeted tests cover:

• missing required attestation blocks destructive flash
• allowed external attestation permits the plan
• internal Apple NVMe attestation is blocked by default
• JSON/Pkl transport parsing for attestation fields

[cwilson613]

Local review complete. Verified the PR only changes src/forge.rs and adds the target-attestation safety gate, Pkl/JSON normalization, and focused tests. Local validation passed: cargo fmt -- --check, cargo clippy --all-targets -- -D warnings, cargo test target_attestation -- --nocapture, and cargo test. GitHub checks are green. No blocking findings.\n\nNote: GitHub rejected an approve review because this is my own PR.