v2 release prep: installer, README, packaging, notes; retire the v1 scripts

Installer rewritten around a preflight: distro, package manager, display
manager, window manager, terminal, tmux, cargo, git, screen locker, touch
device, device permissions and free disk are all checked BEFORE anything is
installed, and the total cost is printed once for a single confirmation.
Prompts read /dev/tty so they still work when the script is piped from curl,
and fall back to defaults with a notice when there is no terminal at all.

Several "[ test ] && action" statements were set -e landmines: under set -e an
AND-OR list that ends up false aborts the script, so a box with no lightdm, no
i3 or nothing to install would have exited silently partway through detection
-- which is exactly the fresh-Debian case the installer exists for. Rewritten
as if-statements and verified against a stripped PATH with no tmux, cargo, git
or package manager present. Also fixed cargo detection reporting blank instead
of NOT INSTALLED: the status of `cargo --version | cut` is cut's, and cut
succeeds on empty input, so the fallback never fired.

Device access now defaults to a udev rule matching touchscreens only, rather
than the input group, which grants access to every input device including the
keyboard and needs a full logout.

README rewritten for someone who has not seen the project: what the photo
shows, the hardware, install, then a config built up step by step, each step
with the YAML and the resulting map. Every example is verified verbatim
against the binary, and every relative link resolves. The mechanism and the
reasoning move to notes/: DESIGN.md, HARDWARE-NOTES.md, V1-BASH.md, TODO.md.

cad/README.md was a verbatim copy of the one inside
geeekpi_rack_adapter_release_v1/, so every path in it -- including the
screenshot -- was broken from where it sits. Corrected to its own level, and
it now states once that the 9-inch screen, the 10-inch mini-rack mount and the
19-inch rack are three different measurements.

The v1 shell implementation is removed; it stays recoverable at tag v1.2 and
notes/V1-BASH.md carries the setting-by-setting migration table.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

notes/TODO.md

@@ -0,0 +1,31 @@
+# TODO
+
+## Before calling v2.0 done
+
+- [ ] **Wyse 3040 validation.** Debian minimal to working rack display,
+      following only the README. Every stumble is a README fix, then re-run.
+      This is the acceptance gate, not a nice-to-have.
+- [ ] Switch the LattePanda over from v1. Its v2 config path and session name
+      differ, so both can be installed side by side while testing.
+- [ ] Add the tested-hardware table to the README once the Wyse is done.
+- [ ] Decide on the position indicator (`indicator: true`). Implemented; keep or
+      cut based on whether it actually helps on the wall.
+
+## Open questions from the plan
+
+- **Snap tie-break.** From `0x2`, with cells at `-1x1` and `-1x3` both at
+  distance 1, the lower column wins. Only fires on the first entry into a row.
+- **Indicator.** Off by default. See above.
+
+## Later
+
+- Prebuilt binaries. Deliberately not in v2.0: source-only until the Wyse install
+  shows how bad a build on an Atom really is. `[package.metadata.deb]` is already
+  in `Cargo.toml`, so `cargo deb` is the cheap next step if it turns out to hurt.
+- `unifly` parameters. `site` and `controller` are accepted by the config and
+  passed as `--site` / `--controller`, but the fork does not implement them yet.
+  Verify against the flags it actually grows.
+- A second monitor type that needs a multi-pane sub-sequence would prove the
+  `Vec<Pane>` model generalises. Nothing needs it yet.
+- SIGTERM handling to remove the control socket on exit. Not urgent: a stale
+  socket is detected and replaced on the next start, which is tested behaviour.