feat(core): add state manifest models

[lynnswap]

Purpose

Add the state and manifest foundation for the PrivateHeaderKit rewrite on top of main.

Changes

• Add manifest and run record models for persisted rewrite state.
• Add target, run-local, and phase status enums required by the rewrite flow.
• Add validated relative artifact paths that reject absolute paths, empty paths, and traversal components.
• Add pretty JSON encode/decode/read/write helpers with exact Date round-trip precision.
• Preserve and validate required nullable JSON fields instead of silently omitting or accepting missing keys.
• Record run execution metadata needed to distinguish simulator/runtime/device/helper context for resume and reproducibility.
• Add run history retention logic for keeping the latest 10 runs.
• Expose the documented top-level generatePrivateHeaders(...) forwarding entry point.
• Cover manifest/run JSON round-trips, artifact path validation, exact timestamp preservation, required nullable keys, execution metadata, and retention ordering.

Verification

• git diff --check
• swift test (93 tests passed)
• codex-review against main (clean, no findings)

Notes

This PR intentionally does not implement CLI wizard flow, cleanup/deletion executors, staging commits, simulator helpers, or legacy marker compatibility.

[lynnswap]

Superseded by #30. The useful state/manifest contracts from this slice are included in the final rewrite foundation PR.