Phase 1d: AutoShrinkEntry registry + green reporting one-liner

[zackees]

Part of #493 Phase 1. Lands the per-platform shrinker registry type and the green auto shrinking: … one-liner reporter — both with empty data so the user-facing behavior is silent on every platform until subsequent phases populate the registry.

Scope

• Define AutoShrinkEntry { category: &'static str, symbols: &'static [&'static str] } in crates/fbuild-build/src/shrink/registry.rs.
• Define a per-platform registry function registry_for(platform: Platform) -> &'static [AutoShrinkEntry] that returns &[] for every platform. Subsequent phases populate this per-platform.
• Implement reporting::print_auto_shrinking_line(entries: &[AutoShrinkEntry]) in crates/fbuild-build/src/shrink/reporting.rs:
  • If entries.is_empty(): print nothing.
  • Otherwise: print a single line in green (via owo-colors) of the form auto shrinking: <sym1>, <sym2>, … — the prefix auto shrinking: is bold-green, the comma-separated symbol list is plain green. owo-colors auto-disables on non-tty and respects NO_COLOR, so no extra plumbing is needed.
• Unit tests:
  • Empty registry → nothing printed (capture stdout).
  • Non-empty registry → exact text auto shrinking: vfprintf, vsnprintf (color codes stripped or asserted separately).
  • The green-coloring path is exercised when owo-colors is forced on.

Out of scope

• Real per-platform registry entries — every platform's auto-shrink list stays empty in this phase. Phases 4–6 populate them.
• The libc probe (separate sub-task).
• The auto-resolver (separate sub-task; Auto always resolves to Off in Phase 1).
• Plumbing this reporter into the orchestrator (separate sub-task once the resolver lands).
• Verbose plan output for explicit modes (separate sub-task).

Pass

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