Linux port (x86_64): portable digup CLI + engine #5

Proposal

Port the digup CLI and its engine (DigUpKit + LlamaRuntime) to Linux x86_64, leaving the menubar app macOS-only. Binary releases (tarball/.deb/.rpm, AUR -bin) via GitHub Actions so nobody compiles from source.

Scope

• In: estimate / index / search / status / eval / index-service / worker on Linux; text, code, HTML; PDFs via poppler; ppt/pptx/odp decks via LibreOffice; llama.cpp CPU build.
• Out: DigUpApp (AppKit/SwiftUI/Sparkle stay macOS-only); images/audio/video backends (honest per-file stubs, no crashes); CUDA/Vulkan; aarch64 (second job later).

Verified on Arch Linux x86_64 (Swift 6.4)

• swift build --product digup + full swift test: 64 DigUpKitTests + 6 LlamaRuntimeTests green.
• vectorsMatchTheReference passes on the Linux build: vectors match macOS at >=0.999, so indexes stay cross-platform (vectors v1).
• End-to-end on real folders: indexed ~400 pages/slides (PDFs + decks + code), hybrid meaning+keyword search verified.
• New Linux CI workflow builds, tests, and packages on PRs and releases.

Ask

1. Is a Linux CLI port something you want upstream?
2. Can you (or CI) run swift build && swift test on macOS against the branch? I changed shared files (Package manifest, CLlama links, a pure refactor of PDF prepare, index ordering append) and have no Mac to check regressions. Happy to rework to fit.