docs: define Gentoo HydraOS target and implementation plan

README.md

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+# HydraOS
+
+Gentoo-based host for concurrent virtual desktops. Its default session shows one
+VM fullscreen and switches between guests with keyboard shortcuts. Other host
+desktops may use the same independent VM manager and viewers.
+
+## Status
+
+Under active development, not yet an installation-ready distribution. The target
+is an installable UEFI ISO, first validated on ASUS TUF Dash F15 FX517ZC. Existing
+NixOS and VM disks are not installation or test targets.
+
+1. Independent QEMU/KVM lifecycle with private QMP control.
+2. Minimal labwc session and fullscreen SPICE viewers.
+3. Gentoo rootfs, live ISO, explicit disk installer.
+4. Disposable-disk installation/reboot tests, then physical live-boot checks.
+
+Shared clipboard and directories are opt-in. VM processes survive viewer and
+compositor exits. GPU passthrough and graphics performance require hardware tests;
+a refresh-rate setting does not prove delivered FPS.
+
+- [Session design](docs/superpowers/specs/2026-10-13-graphics-session-design.md)
+- [Implementation plan](docs/superpowers/plans/2026-10-13-graphics-session.md)
+- [Hardware target](docs/hardware/asus-fx517zc.md)
+
+Build outputs, VM disks, runtime state and credentials stay outside Git.