Backport the session log block from the installed copy

/usr/bin/start-hyprcosmic gained this during the blank-screen debugging and
the versioned copy never did, so the next install from this repository would
have silently removed it. Found by diffing the two while writing an installer;
the installer's --check exists so that the next one is not found by hunch.

What it does, unchanged from the copy that has been running: redirect the
session's own output to ~/.cache/hyprcosmic/session.log. A session that fails
drops you at the greeter taking its stderr with it, and by then the journal
cannot be read from inside a session that no longer exists.

A plain redirect rather than tee into a process substitution -- tee would be a
child the session writes through, so its death would deliver SIGPIPE, and
nothing about diagnostics should be able to kill the desktop. Every step is
guarded, so a failure here starts the session with no log rather than refusing
to log in. The previous log is kept as .1 so a second attempt does not erase
the evidence from the first.

data/start-hyprcosmic

@@ -26,6 +26,36 @@ if [[ ! -x "$HYPRCOSMIC_SESSION_BIN" ]]; then
     exit 1
 fi
 
+# Capture this session's output to a file that outlives it.
+#
+# A session that fails drops you straight back to the greeter, taking its
+# stderr with it -- and by then you cannot read the journal from inside a
+# session that no longer exists. A plain file in the cache directory is still
+# there afterwards, whatever happened.
+#
+# A plain redirect, not `tee` into a process substitution: tee would be a child
+# process the session writes through, so if it ever died the session would take
+# a SIGPIPE. Nothing about diagnostics should be able to kill the desktop.
+#
+# Every step is guarded. If any of it fails the session starts anyway with no
+# log -- losing diagnostics is an inconvenience, refusing to log in is not.
+#
+# The previous log is kept as .1, so a second attempt does not erase the
+# evidence from the first.
+HYPRCOSMIC_LOG="${XDG_CACHE_HOME:-$HOME/.cache}/hyprcosmic/session.log"
+if mkdir -p "$(dirname "$HYPRCOSMIC_LOG")" 2>/dev/null; then
+    if [[ -f "$HYPRCOSMIC_LOG" ]]; then
+        mv -f "$HYPRCOSMIC_LOG" "${HYPRCOSMIC_LOG}.1" 2>/dev/null || true
+    fi
+    if : >>"$HYPRCOSMIC_LOG" 2>/dev/null; then
+        exec >>"$HYPRCOSMIC_LOG" 2>&1
+        echo "=== hyprcosmic session starting $(date -Is) ==="
+        echo "    session bin: $HYPRCOSMIC_SESSION_BIN"
+        echo "    comp bin:    $HYPRCOSMIC_COMP_BIN"
+        echo "    autostart:   ${XDG_CONFIG_HOME:-$HOME/.config}/hyprcosmic/autostart"
+    fi
+fi
+
 # Selects the component set in cosmic-session's profile module: cosmic-panel,
 # cosmic-launcher, cosmic-app-library, cosmic-workspaces, cosmic-bg and
 # cosmic-files-applet are skipped; waybar and friends come from