Phase 1c: per-context auto-resolver shell (returns Off for every platform) #504

Part of #493 Phase 1. Lands the resolve_for_context shell that combines the explicit CLI mode (from resolve_explicit in #496) with a ShrinkContext (platform + libc + framework) and produces a per-platform ShrinkMode.

In Phase 1c every Auto resolution returns Off. Real per-platform decisions (newlib + Arduino-ESP32 IDF 5.x → Safe, etc.) land in Phase 4+ as each platform is end-to-end validated.

Scope

• Add ShrinkContext { platform, libc, framework } struct in crates/fbuild-build/src/shrink/.
• Add resolve_for_context(mode: ShrinkMode, ctx: &ShrinkContext) -> ShrinkMode:
  • Explicit modes (Off, Safe, Aggressive, Printf) pass through unchanged.
  • Auto always returns Off for every (platform, libc, framework) combination in Phase 1c.
• Unit tests:
  • Each explicit mode passes through unchanged regardless of context.
  • Auto resolves to Off for every (Platform, Libc) combination.

Out of scope

• Wiring resolve_for_context into the orchestrator — that lands in Phase 4 alongside the first real per-platform shrinker.
• Real per-platform decisions — Phases 4–6 populate the matrix.
• Per-framework probe (Arduino-ESP32 vs ESP-IDF native vs Mbed) — done in Phase 4 when first needed.

Pass

• cargo build --workspace --all-targets clean.
• cargo clippy --workspace --all-targets -- -D warnings exits 0.
• New unit tests pass.