refactor(platform): phase 8/9 migrate deploy, toolchain, emulator, and specialized exceptions

[zackees]

Parent: #1306
Depends on: phase 7 device migration

Scope

Resolve every remaining domain-specific occurrence in deploy, toolchain, package, build, emulator, Python, and tests:

• consume neutral host/executable/device/process/fs facts for host artifacts and launch mechanics;
• keep embedded board/compiler target policy in product code;
• migrate remaining native mechanics into the existing facade;
• for any genuine specialized artifact, enforce an exact named zone and narrow contract rather than a wildcard/file allowlist;
• remove remaining target-specific native dependency ownership outside the boundary;
• delete exact migrated ledger rows.

Acceptance criteria

• RED tests prove host and embedded target choices remain independent.
• Toolchain downloads, deployers, emulators, Python bindings, and build paths contain no unclassified host leakage.
• Every specialized exception is named, justified, narrowly linted, and cross-platform validated.
• Existing deploy/toolchain/emulator behavior and diagnostics remain compatible.
• Before/after ledger totals and RED -> GREEN evidence are recorded.
• Focused tests, three-host CI, workspace check/clippy, and repo tests are GREEN.