Threshold at 1.5x doubles the tap rate; add coverage-guided enrolment

The threshold change is the first thing to move accuracy. Labelled protocol,
enrolled quick taps: 4 of 10, against 2 of 10 for every previous build. Seven
of fifteen presses now get two or more frames where none did before, and the
landing frames the old threshold discarded turn up in the transcript at 207,
213, 242 and 284. Wrong finger still 0 of 5.

It also explains the latency asymmetry Jorijn noticed. A match ends the loop
the moment it happens, mean 1317 ms. A no-match waits for the finger to lift,
mean 2190 ms, because a later frame in the same press may still match -- and
they do, at frames 3, 5 and 8. That is the press rule working, not a fault.

What the same run says about the template is worse than the loop ever was. One
press produced eight frames at full contact, 362 to 367, and every one
rejected; another produced six. Roughly one frame in seven matches, against the
47 to 75 percent the journal records for the template the research harness
enrolled. The first template here was ten quick taps in about one position, so
the algorithm was handed ten near-duplicate images of one spot.

fpenrol.sh enrols with a different contact position per ACCEPTED sample --
centre, left, right, tip, joint, rolled either way -- driven by
fprintd-enroll's own per-stage output rather than by counting presses. Enrol as
a SECOND finger so both templates coexist and the comparison holds the loop
constant; the trustlet reports which fid matched, and the daemon's map turns
that back into a name.

packaging/fpenrol.sh

@@ -0,0 +1,48 @@
+#!/bin/sh
+# fpenrol.sh -- enrol a finger with COVERAGE guidance.
+#
+#   fpenrol.sh [finger_name]        default right-middle-finger
+#
+# The first template on this device was enrolled from ten quick taps in
+# roughly one position, and it matches roughly one frame in seven -- against
+# 47-75% for the template the research harness enrolled. The algorithm was
+# given ten near-duplicate images of one spot, so it recognises that spot and
+# little else.
+#
+# This walks a different contact position per accepted sample. It reads
+# fprintd-enroll's per-stage output and prompts the next position as each
+# stage passes, so the guidance tracks what the trustlet ACCEPTED rather than
+# how many times you pressed.
+F=${1:-right-middle-finger}
+i=1
+set -- "centre of the pad" \
+       "slightly LEFT of centre" \
+       "slightly RIGHT of centre" \
+       "higher up, toward the TIP" \
+       "lower down, toward the JOINT" \
+       "centre again, rolled LEFT" \
+       "centre again, rolled RIGHT" \
+       "tip, rolled slightly left" \
+       "joint, rolled slightly right" \
+       "centre, pressed a little firmer"
+echo "enrolling $F with position guidance -- 10 samples"
+echo "press FLAT and firm, hold about a second, then LIFT fully."
+echo
+# The reader runs in a subshell, so `shift` there cannot advance the caller's
+# list. Index into it instead.
+pos() { eval "printf '%s' \"\$$1\""; }
+echo ">>> sample 1/10: $(pos 1)"
+fprintd-enroll -f "$F" user 2>&1 | while IFS= read -r line; do
+    case "$line" in
+        *enroll-stage-passed*)
+            i=$((i+1))
+            if [ "$i" -le 10 ]; then
+                echo "    accepted."
+                echo ">>> sample $i/10: $(pos $i)"
+            fi ;;
+        *enroll-retry-scan*)  echo "    not accepted -- same position, press flatter and firmer" ;;
+        *enroll-completed*)   echo; echo "COMPLETED." ;;
+        *enroll-failed*)      echo; echo "FAILED." ;;
+        *) printf '%s\n' "$line" ;;
+    esac
+done