refactor(platform): phase 4/9 migrate process and containment mechanics #1310

Parent: #1306
Depends on: phase 3 host/executable facts

Scope

Move fbuild-owned process mechanics behind fbuild_core::platform::process:

• command/background configuration and Windows console suppression;
• containment, process groups/job objects, parent-death behavior, termination and exit interpretation;
• PID liveness and executable-image inspection;
• daemon/CLI subprocess lifecycle primitives.

Delegate to running-process wherever its neutral API already owns the behavior. Do not duplicate running-process. Keep product retry policy and diagnostics in callers. Delete the migrated ledger rows atomically.

Acceptance criteria

• RED characterization covers containment, termination, PID/image inspection, and platform command setup.
• Shared callers expose no raw handle/fd, native process API, or concrete implementation.
• Existing daemon crash recovery and child cleanup contracts remain compatible.
• Before/after ledger totals and RED -> GREEN evidence are recorded.
• Focused process tests, three-host CI, workspace check/clippy, and repo tests are GREEN.