refactor(platform): phase 9/9 reach zero baseline and make enforcement permanent

[zackees]

Parent: #1306
Depends on: phases 1-8

Scope

Perform the final requirement-by-requirement audit and consolidate the zero-debt end state:

• remove every remaining transitional ledger row;
• delete ledger regeneration/dump escape hatches;
• retain the permanent pre-expansion Dylint, whole-tree scanner, manifest checker, and zero-count test;
• prove exactly one production cfg_select! selector remains;
• prove all native dependencies and concrete modules are confined to the authorized boundary/specialized zones;
• run complete Windows, Linux, and macOS validation across representative capabilities;
• reconcile documentation and remove transitional adapters.

Acceptance criteria

• RED proof shows reintroducing any forbidden occurrence fails.
• Transitional baseline and regeneration path are deleted.
• Permanent enforcement reports zero unclassified occurrences over all handwritten Rust sources.
• Exactly one production selector remains and has no fallback.
• No new workspace crate exists and no temporary adapter remains.
• Full repository validation and required three-host CI are GREEN.
• The PR records the final audit against every refactor(platform): centralize host mechanics behind fbuild-core::platform and cfg_select! #1306 acceptance criterion.
• After merge, all phase issues are closed and refactor(platform): centralize host mechanics behind fbuild-core::platform and cfg_select! #1306 can be closed with verified evidence.