Phase 1a: ShrinkMode enum + --shrink / --no-shrink CLI flags #496

Part of #493 Phase 1. The smallest cohesive next chunk — add the CLI surface and ShrinkMode type without wiring any behavior change.

Scope

• Define ShrinkMode { Auto, Off, Safe, Aggressive, Printf } in crates/fbuild-build/src/shrink/mod.rs with Default = Auto and clap::ValueEnum.
• Add --shrink[=MODE] (with default_missing_value = "auto") and --no-shrink (with conflicts_with = "shrink") to:
  • The global Cli struct in crates/fbuild-cli/src/cli/args.rs (matches --clean / --environment precedent).
  • The Build, Deploy, TestEmu, Ci subcommands.
• Resolve to a single ShrinkMode value per the precedence: global → subcommand → --no-shrink always wins.
• Plumb the resolved ShrinkMode through the build entry point as a function parameter (currently unused).

Out of scope

• Libc probe (separate sub-task).
• Auto-resolver per-platform decisions (separate sub-task; always returns Off in Phase 1).
• Reporting (separate sub-task — green one-liner, verbose plan).
• build_info.json shrink record (separate sub-task).
• Any actual link-flag emission, shadow archive, spec file (Phases 2–4).

Pass

• cargo build --workspace --all-targets clean.
• cargo clippy --workspace --all-targets -- -D warnings exits 0.
• Unit test: clap parses --shrink, --shrink=auto, --shrink=off, --shrink=safe, --shrink=aggressive, --shrink=printf, --no-shrink correctly on the Build subcommand.
• Unit test: --shrink + --no-shrink together is a clap conflict error.
• Integration: fbuild build tests/platform/esp32s3 -e esp32s3 --shrink=safe and --no-shrink both produce byte-for-byte identical firmware to the baseline (no link behavior change yet).