Phase 1e: ShrinkRecord struct for build_info.json telemetry #506

Part of #493 Phase 1. Lands the ShrinkRecord and Strategy types that will eventually be embedded in build_info_<env>.json so fbuild bloat and CI bloat-budget gates can label per-symbol reports with the shrink decision that produced them.

In Phase 1e the types are defined and round-trip through JSON; actual emission into build_info.json lands when an orchestrator first calls resolve_for_context end-to-end (Phase 4).

Scope

• Add #[derive(Serialize, Deserialize)] + #[serde(rename_all = "kebab-case")] to ShrinkMode so JSON output uses the same kebab-case names as the CLI (auto, off, safe, aggressive, printf).
• Add crates/fbuild-build/src/shrink/record.rs:
  • pub enum Strategy { SpecFile, Wrap } with kebab-case JSON (spec-file / wrap).
  • pub struct ShrinkRecord { mode: ShrinkMode, resolved: ShrinkMode, applied: Vec<String>, strategy: Option<Strategy> }.
  • strategy serializes as null / omitted when not present.
  • applied defaults to empty.
• Wire pub mod record; into shrink/mod.rs.
• Unit tests:
  • ShrinkMode round-trips through JSON for every variant.
  • Strategy round-trips for both variants.
  • ShrinkRecord defaults to all-Auto / empty / None and round-trips.
  • ShrinkRecord with a populated applied list and Strategy::SpecFile round-trips.

Out of scope

• Emitting ShrinkRecord into build_info.json — wiring happens when an orchestrator first calls resolve_for_context (Phase 4).
• Reading ShrinkRecord from build_info.json in fbuild bloat — separate task once the emission lands.

Pass

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