(3/5) The Linux host, and the devices calling what they connect to a host

[ErickXavier]

The third of four pull requests that bring Sill to more systems, each on the one before; each builds and tests on its own. It needs the previous one merged first: until then, its diff also shows the earlier branches' commits, and only its last commits are its own.

1. feat/android-client: the Android client, Keep Screen On, and Sill.app's device words
2. feat/windows-client: the Windows client
3. feat/linux-host: the Linux host, and the devices calling what they connect to a host ← this one
4. feat/windows-host: the Windows host

The site has its own pull request (docs/site-other-platforms), independent of these four.

A Linux PC on X11 streams its own windows to the same iPhone, iPad, Android and Windows apps, over Sill for Mac's wire, TLS door and pairing. No client changes for it.

It has run only against the Kotlin client, never with an iPhone, iPad or Android app. docs/clients-plan.md lists L1–L3, and docs/ui-parity.md lists H8–H10.

What it adds

• clients/host-core (JDK 17): the door, pairing and session of any host that isn't the Mac, over a HostPlatform.
  • The door: TLS 1.3, both ALPNs, a TLS context per connection, and the Mac's pending caps and refusal backoff.
  • The Mac's pairing window and ask rules (kind 21 included), the trust list, and an mDNS responder.
  • Sending to each device with the Mac's in-flight, keyframe and eviction rules.
  • The coordinator: catalog order, select, viewport and Aa, kinds 16/17/18, clipboard, gestures, the PC's pointer as kind 26, goodbyes.
  • The Mac's input made the PC's. A key's or button's up with nothing streaming is still delivered.
  • Sill.app's StatusText and MenuBuilder rules, and a QR encoder identical to python-qrcode's.
• clients/host (JDK 25, fetched by Gradle):
  • LinuxPlatform: Xlib and XTest through the JDK's foreign function API.
  • FfmpegVideo: x11grab, then NVENC, Quick Sync or VAAPI, else x265.
  • Sill.app's status card and menu as a tray popup, plus Settings, the pairing window with its QR code, and the log.
  • The clipboard never sends a copy a password manager marks as concealed.
  • Flags: --headless, --pair, --self-test, --input-probe.
• Words: the host's say "device" and "this PC", never only iPhone and iPad.
• The devices call what they connect to a host, not a Mac (its own commit): with a PC as a host, "Connect to a Mac" and "Add a Mac…" are wrong.
  • The iOS app, the core's words and the Android and Windows clients say "Connect to a Host", "Add a Host…", "Looking for hosts…", "your host" and "the host". A host that gives no name is "Host", and a host's own name shows wherever it's known, as before. Comments and console lines keep "Mac".
  • The checks that hold these words (home-device, policy, goodbye, home-model, remote-rules, menu-state, tour) and clients/design's anchors follow, and check_copy.py still finds the core's words in the iOS app.
  • Sill.app's and the PC host's pairing windows say "Add a Host… (Add a Mac… in older versions)", for devices still on 0.5.1.
  • The App Store's screenshots still show "Connect to a Mac" until new ones are taken.
• FFmpeg's GPL build is the only one with a software HEVC encoder, so a packaged host is distributed under GPLv3 while the source stays Apache-2.0. docs/BRIEF.md records the decision.
• The code review's host fixes, each with a test that fails without it:
  • a TLS ticket from a pairing connection no longer resumes as a full session (HostDoorTest: 3 of 3 let in before);
  • LaunchApp only opens a listed app;
  • kind 21 goes through AskLimits;
  • a trusted key's ask gets a code;
  • a window partly off the screen is clicked against what is captured (CaptureRect).
• Not built: the Mac's menus, the virtual display, Direct Wireless, Wayland-native windows, multiple monitors.
• Code left for the next PR: a few Windows branches stay in shared files (HostStore's folder, Theme's dark mode, the encoder list), dormant until the Windows platform arrives in the next pull request.
• CI: clients.yml's hosts-linux job runs the tests under Xvfb with openbox and xterm, then the packaged host's self-test. It keeps the UI screenshots.