Build PrivateHeaderKit rewrite foundation

[lynnswap]

Purpose

• Provide the final review branch for the PrivateHeaderKit rewrite against codex/rewrite-final-base, which was created from main at dc744389e9e2c03c4d647491ec06aafc9a9a3d57.
• Replace the old public dump-command architecture with one public privateheaderkit command and a Core-owned, resume-safe generation pipeline.

Changes

• Add the new generation model in PrivateHeaderKitCore: source labels, output/state layout, target discovery/resolution, resume decisions, manifest/run JSON, managed artifact cleanup, run locking, and raw-dump invocation contracts.
• Make privateheaderkit the primary user-facing entry point. Running it without arguments starts interactive generation; automation passes generation flags directly to privateheaderkit without a public generate subcommand.
• Keep install as a hidden/bootstrap path for swift run -c release privateheaderkit install; it is not listed as a normal installed command choice.
• Keep user-facing target selection as comma-separated target queries instead of stable target IDs, while preserving structured ambiguity/no-match results for UI flows.
• Consolidate public execution to one installed privateheaderkit command. Old command names are intentionally rejected, and low-level extraction is hidden behind privateheaderkit __raw-dump.
• Add iOS simulator generation support through the internal privateheaderkit-sim-helper, installed under libexec, with simctl spawn execution and runtime/device resolution.
• Rename and retain the raw dump engine as internal PrivateHeaderKitRawDumpCore / PrivateHeaderKitRawDumpRuntimeObjC modules, removing legacy public dump targets and stale orchestration code.
• Adapt Swift interface generation to upstream SwiftIndexEvents and switch Mach-O dependencies back to upstream MxIris sources where possible, including direct upstream MachOSwiftSection.
• Delete Docs/RewriteRequirements.md after applying the rewrite decisions.

Testing

• swift test --force-resolved-versions
• git diff --check
• swift run -c release privateheaderkit install
• privateheaderkit --help
• printf '14\n\n' | privateheaderkit to verify the interactive flow shows the default output directory without asking for an output path

Notes

• This remains a draft PR for final user review.
• Intermediate implementation PRs merged into this branch include simulator support, runtime bound handling, state locking, and upstream Swift index event adaptation.
• codex-review is intentionally held until requested by the user.