Phase 0a: scaffold shrink module + add owo-colors dep #494

Part of #493 (Phase 0). The smaller half — does not depend on #491.

Scope

• Add owo-colors = "4" to crates/fbuild-cli/Cargo.toml (auto-disables on non-tty + respects NO_COLOR).
• Create crates/fbuild-build/src/shrink/mod.rs — empty module stub with module doc.
• Create crates/fbuild-build/src/shrink/README.md — placeholder describing what the module will contain.
• Wire pub mod shrink; into crates/fbuild-build/src/lib.rs.

Out of scope

• fbuild bloat: emit and preserve firmware.map automatically #491 (firmware.map auto-emit) — separate, larger sub-task tracked in its own issue.
• Any actual shrink logic — Phase 1+ work.

Pass criteria

• cargo build --workspace --all-targets clean.
• cargo clippy --workspace --all-targets -- -D warnings clean.
• No public API changes; no behavior change.

Why split it out

Phase 0 in #493 bundles "land #491" with the trivial scaffolding. #491 is its own substantive linker-flag work across all platform orchestrators; the scaffolding can ship today and unblock Phase 1 work in parallel.