fix(mime): text/plain falls through to Text Editor on live (#119)

A singleton default naming code.desktop left the live ISO with no text
handler — GIO skips missing .desktop files and offers nothing. Prefer
vscode when present, then org.gnome.TextEditor.desktop. Live also gains
mpv (video defaults had the same hole; free via the template pin).

checks.live-baseline-apps now requires every Default Applications key
to resolve to a present .desktop on live (HM + system path) and on the
default template install. Proved to fail on a vscode-only text/plain.

Verified: V2 (live-baseline-apps green + prove-to-fail); V0 flake check.

flake.nix

@@ -560,23 +560,37 @@
                 touch $out
               '';
 
-          # Live-ISO baseline apps (ROADMAP § live-ISO baseline apps, #103).
-          # The live session shipped no
-          # browser and no office for as long as it existed — the thing a user
-          # judges the distro by before installing, and boots to rescue a dead
-          # machine. Nothing in a build fails when an app silently leaves the
-          # live profile (it just stops being in the launcher, which is
-          # invisible until someone boots the ISO on real hardware and looks),
-          # so assert both halves: on PATH *and* a .desktop the launcher can
-          # see. Same artifacts the ISO ships, no ISO build.
+          # Live-ISO baseline apps (ROADMAP § live-ISO baseline apps, #103)
+          # plus the mime-default presence guard (#94 / #119). The live
+          # session shipped no browser and no office for as long as it
+          # existed — the thing a user judges the distro by before installing,
+          # and boots to rescue a dead machine. Nothing in a build fails when
+          # an app silently leaves the live profile (it just stops being in
+          # the launcher), and a mime default that names a missing .desktop is
+          # silently skipped by GIO (no handler at all) — so assert both:
+          # binaries + launcher entries for the baseline suite, and that
+          # every Default Applications key has at least one listed .desktop
+          # present in the live profile or system path (thunar is system-side).
+          # Same artifacts the ISO ships, no ISO build.
           live-baseline-apps =
             let
               liveHm =
                 self.nixosConfigurations.nomarchy-live.config.home-manager.users.${username};
               gen = liveHm.home.activationPackage;
+              # system.path carries NixOS-side apps (thunar); HM-only
+              # home-path does not. A mime default that only names a
+              # system desktop must still resolve.
+              sysPath =
+                self.nixosConfigurations.nomarchy-live.config.system.path;
+              # Template install path: same mime defaults, different package
+              # set (vscode present, gnome-text-editor not). Guard both so
+              # a live-only fix cannot re-break the installed machine.
+              templateGen = downstream.homeConfigurations.me.activationPackage;
+              templateSys =
+                downstream.nixosConfigurations.default.config.system.path;
             in
             pkgs.runCommand "nomarchy-live-baseline-apps"
-              { nativeBuildInputs = [ pkgs.gnugrep pkgs.findutils ]; }
+              { nativeBuildInputs = [ pkgs.gnugrep pkgs.findutils pkgs.python3 ]; }
               ''
                 set -euo pipefail
                 apps=${gen}/home-path/share/applications
@@ -590,11 +604,12 @@
                          impress.desktop \
                          org.gnome.TextEditor.desktop \
                          io.bassi.Amberol.desktop \
+                         mpv.desktop \
                          org.gnome.Snapshot.desktop; do
                   test -f "$apps/$d" || { echo "live-baseline-apps: MISSING launcher entry $d"; exit 1; }
                 done
 
-                for b in chromium libreoffice gnome-text-editor amberol snapshot; do
+                for b in chromium libreoffice gnome-text-editor amberol mpv snapshot; do
                   test -x "$bins/$b" || { echo "live-baseline-apps: MISSING binary $b"; exit 1; }
                 done
 
@@ -603,16 +618,67 @@
                 # so GIO silently skipped the entry and the live session had
                 # no default browser at all).
                 mimes=${gen}/home-files/.config/mimeapps.list
-                grep -q '^x-scheme-handler/https=chromium-browser.desktop' "$mimes"
+                grep -q '^x-scheme-handler/https=chromium-browser.desktop' "$mimes" \
+                  || { echo "live-baseline-apps: https mime must name chromium-browser.desktop (#94)"; exit 1; }
                 test -f "$apps/chromium-browser.desktop"
 
+                # text/plain must prefer vscode but fall through to the live
+                # editor (#119) — never a singleton that only names code.
+                grep -qE '^text/plain=.*org\.gnome\.TextEditor\.desktop' "$mimes" \
+                  || { echo "live-baseline-apps: text/plain must fall through to org.gnome.TextEditor.desktop (#119)"; exit 1; }
+                test -f "$apps/org.gnome.TextEditor.desktop"
+
+                # Every Default Applications key: at least one listed
+                # .desktop must exist in home-path OR system path. A key
+                # whose every entry is missing is the #94/#119 bug class.
+                # Covers live *and* the default template install.
+                python3 - "$mimes" "$apps" "${sysPath}/share/applications" \
+                         "${templateGen}/home-files/.config/mimeapps.list" \
+                         "${templateGen}/home-path/share/applications" \
+                         "${templateSys}/share/applications" <<'PY'
+                import sys
+                from pathlib import Path
+
+                def check(label, mimes, *app_dirs):
+                    text = Path(mimes).read_text()
+                    in_defaults = False
+                    bad = []
+                    for line in text.splitlines():
+                        if line.startswith("["):
+                            in_defaults = line.strip() == "[Default Applications]"
+                            continue
+                        if not in_defaults or not line.strip() or line.startswith("#"):
+                            continue
+                        if "=" not in line:
+                            continue
+                        key, val = line.split("=", 1)
+                        desktops = [d for d in val.split(";") if d.strip()]
+                        if not desktops:
+                            bad.append(f"{key}= (empty)")
+                            continue
+                        if not any(
+                            any((Path(d) / desk).is_file() for d in app_dirs)
+                            for desk in desktops
+                        ):
+                            bad.append(f"{key}={';'.join(desktops)} (none present)")
+                    if bad:
+                        print(f"live-baseline-apps: mime defaults missing on {label}:")
+                        for b in bad:
+                            print(f"  {b}")
+                        sys.exit(1)
+                    print(f"live-baseline-apps: mime defaults ok on {label}")
+
+                check("live", sys.argv[1], sys.argv[2], sys.argv[3])
+                check("template", sys.argv[4], sys.argv[5], sys.argv[6])
+                PY
+
                 # Firefox is deliberately absent — a second browser is the one
                 # item in #103's list that costs a real closure, and chromium
                 # owns the mime default (Bernardo 2026-07-14). If it ever
                 # ships here, that is a size decision, not a drive-by.
                 test ! -f "$apps/firefox.desktop" || { echo "live-baseline-apps: firefox reappeared — size decision, see #103"; exit 1; }
 
-                echo "live-baseline-apps: ok — browser, office, editor, music, camera on PATH + in the launcher"
+                echo "live-baseline-apps: ok — browser, office, editor, music, video, camera + mime defaults resolve"
                 touch $out
               '';
 
@@ -1919,6 +1985,8 @@
                   libreoffice-fresh   # documents/sheets/slides off a dying disk
                   gnome-text-editor   # the maintained gedit successor
                   amberol             # music player (also in the template)
+                  mpv                 # video (mime defaults → mpv.desktop; also
+                                      # template — free on the ISO via the pin)
                   snapshot            # camera — the maintained Cheese successor
                 ];
               };