Adopt stock's enrolment-quality config: 20 samples, and thresholds that refuse a bad one

Jorijn asked whether the sample count is ours to control and whether more
samples would help. It is -- and the config dump says something more useful.
Stock enrols with 20 samples, not the 10 we had guessed, but it also sets six
keys we were leaving entirely to the trustlet's built-in defaults:

    min_enrolling_coverage_threshold   70
    min_enrolling_quality_threshold    20
    enroll_overlap_min_area            60
    enroll_overlap_max_area            80
    enable_duplicated_finger_checking  on
    max_extral_enroll_low_quality      16

Those defaults accept anything, which is how ten quick taps in one position
became a stored template. The overlap bounds are the mechanism that matters:
a sample must overlap the previous one by 60 to 80 percent -- more means the
finger did not move, less means a gap -- so the algorithm now refuses the
same-spot sample instead of storing it. Sample count alone would have stored
twenty near-duplicates rather than ten.

Deliberately left out: min_identify_quality_threshold and
min_identify_coverage_threshold. Those gate matching rather than enrolment and
would reject exactly the landing frames the 1.5x detection threshold exists to
catch. One group of variables at a time.

fpenrol.sh grows to twenty positions, and its rejection message now says to
move rather than to press harder, because the enforcement and the guidance
finally agree on what a rejected sample means.

packaging/fpenrol.sh

@@ -7,7 +7,9 @@
 # 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.
+# little else. It accepted them because our config set none of stock's
+# enrolment-quality keys and the trustlet's own defaults accept anything;
+# those keys are now set, so the algorithm enforces the spread too.
 #
 # This walks a different contact position per accepted sample. It reads
 # fprintd-enroll's per-stage output and prompts the next position as each
@@ -15,32 +17,46 @@
 # how many times you pressed.
 F=${1:-right-middle-finger}
 i=1
+# Twenty positions, stock's sample count. The config now enforces stock's
+# coverage and overlap thresholds, so a sample that does not move enough from
+# the last one is REFUSED by the algorithm rather than stored -- the guidance
+# and the enforcement finally agree.
 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" \
+       "centre, rolled LEFT" \
+       "centre, rolled RIGHT" \
        "tip, rolled slightly left" \
        "joint, rolled slightly right" \
-       "centre, pressed a little firmer"
-echo "enrolling $F with position guidance -- 10 samples"
+       "centre, a little firmer" \
+       "far LEFT edge of the pad" \
+       "far RIGHT edge of the pad" \
+       "very tip of the finger" \
+       "well down toward the joint" \
+       "upper left of the pad" \
+       "upper right of the pad" \
+       "lower left of the pad" \
+       "lower right of the pad" \
+       "centre, rolled hard left" \
+       "centre, rolled hard right"
+echo "enrolling $F with position guidance -- 20 samples (stock's count)"
 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)"
+echo ">>> sample 1/20: $(pos 1)"
 fprintd-enroll -f "$F" user 2>&1 | while IFS= read -r line; do
     case "$line" in
         *enroll-stage-passed*)
             i=$((i+1))
-            if [ "$i" -le 10 ]; then
+            if [ "$i" -le 20 ]; then
                 echo "    accepted."
-                echo ">>> sample $i/10: $(pos $i)"
+                echo ">>> sample $i/20: $(pos $i)"
             fi ;;
-        *enroll-retry-scan*)  echo "    not accepted -- same position, press flatter and firmer" ;;
+        *enroll-retry-scan*)  echo "    REFUSED -- move further from the last position, or press flatter" ;;
         *enroll-completed*)   echo; echo "COMPLETED." ;;
         *enroll-failed*)      echo; echo "FAILED." ;;
         *) printf '%s\n' "$line" ;;