refactor(platform): phase 1/9 research gate and three-host inventory

[zackees]

Parent: #1306

Context

This phase establishes the evidence required before fbuild changes ownership or freezes a platform-debt baseline. The preliminary scan in #1306 is intentionally not authoritative.

Scope

• Add a reviewed architecture note for the single fbuild_core::platform boundary.
• Inventory every handwritten Rust host cfg, native API/path, concrete platform reference, compile-time host fact, and target-specific native dependency on Windows, Linux, and macOS; reconcile the union.
• Classify each occurrence as host mechanic, host-artifact policy, embedded build-target policy, generated/vendor source, or narrowly justified specialized artifact.
• Define neutral APIs for process, fs, IPC, executable, host, and device without leaking native types.
• Prove the fbuild-core dependency placement is cycle-safe and conforms to the no-new-crate rule.
• Enumerate every fbuild-owned Rust 1.94.1 pin that phase 2 must update and distinguish intentional user-toolchain fixtures.
• Record focused RED evidence showing that private, inactive, and test-only violations currently pass.
• Confirm or amend phases 3-8 using inventory dependency edges.

Acceptance criteria

• Three-host reconciled inventory and architecture note are merged.
• Every occurrence and exceptional component has a documented classification.
• Host mechanics, host-artifact policy, and embedded targets are explicitly distinguished.
• RED evidence is preserved in the PR.
• No production ownership or toolchain pin changes in this phase.
• Validation commands and the authoritative inventory totals are recorded before close.