[Foundation] FT-3: Convert public static build properties to BuildContext-backed facades

[ChrisonSimtian]

Summary

Convert the public static build properties to thin facades over BuildContext.Current, and stop the engine internals from reading the statics directly. Backwards-compatible: the public API is preserved.

Findings (file:line)

FalloutBuild.Statics.cs — these are public static and set once in the static ctor:

• RootDirectory (:36), TemporaryDirectory (:41), BuildAssemblyFile (:46), BuildAssemblyDirectory (:51), BuildProjectDirectory (:56), BuildProjectFile (:61), Verbosity (:67, mutable), Host (:77, mutable), LoadedLocalProfiles (:80).

Scope

• Re-express each as => BuildContext.Current.<…> (facade).
• Replace direct static reads inside Execution/ with ctx.<…>.

Acceptance criteria

• No FalloutBuild.<static> reads remain inside the Execution/ namespace.
• Public API signatures unchanged — verify against the PublicApi snapshot (build/Build.PublicApi.cs).
• Non-breaking.

Notes

• Important: these statics are public API, so removing them would be a breaking change (batched to the year cut per CLAUDE.md rule Enterprise hardening: repo nuget.config breaks corporate packageSourceMapping setups #2). The facade approach keeps this non-breaking — do NOT remove them.
• Depends on: FT-2.
• Size: M.