refactor(platform): phase 2/9 bootstrap cfg_select boundary, Rust 1.95, and exact-occurrence ratchet #1308

Parent: #1306
Depends on: phase 1 research gate

Context

Use the reviewed phase 1 inventory as the only input to the transitional baseline. This phase creates the boundary and enforcement but does not migrate a capability.

Scope

• Raise every fbuild-owned build/MSRV pin from Rust 1.94.1 to one selected 1.95.x release, with drift tests; preserve intentional arbitrary-toolchain fixtures.
• Add fbuild_core::platform with exactly one std::cfg_select! selector for Windows, Linux, and macOS and no fallback.
• Add neutral facade indexes and private concrete roots with required READMEs; do not add a workspace crate.
• Add a pre-expansion enforce_platform_boundary Dylint and an independent whole-tree/manifest checker.
• Freeze the reconciled exact-occurrence ledger. New, stale, duplicate, malformed, or growing entries and scanner/Dylint disagreement must fail CI.
• Add forbidden and allowed fixtures, including private/inactive/test code and a second identical occurrence in a grandfathered file.
• Wire all checks into existing Dylint and normal CI entrypoints and document the boundary.

Acceptance criteria

• RED evidence for missing selector/toolchain/enforcement is recorded.
• Rust 1.95.x declarations and drift coverage are complete.
• The single selector and six neutral namespaces compile on all supported hosts.
• Exact-occurrence baseline is frozen from phase 1 and cannot silently grow.
• Dylint, scanner, manifest checker, targeted tests, workspace check/clippy, and repo tests are GREEN.
• No capability implementation has moved yet.