snapshots: measure SELECT / READ commands and show timing stats

Server measures every classified APDU (A4 select, B0 read binary, B2 read
record) from command to response: _collect_apdu_times() enables the tracer
(reattaching it if pySim nulled it) and /api/select + /api/read return
'apdu_times': [{type, ms}]. Collection is safe: handlers hold _CARD_LOCK.

Snapshots store per-file {select_ms, read_ms}, a ms value per record and
snapshot-level stats {select, read_binary, read_record}: {min, max, avg,
count} plus total_ms (wall time of the scan). The snapshot view gets a
summary under the title (files/records counts, scan time, min/avg/max per
command type) and shows select/read per file and read time per record.
Timings are display-only: checks, snapshots comparison and imports ignore
them (old snapshots simply show 'No timing data').

Tests: Python classifier/collection (tests/test_apdu_timing.py) and
frontend stats/accumulator/format/build-file/summary. SW cache v99 ->
v100; help, README and docs/api.md updated.

frontend/help.html

@@ -394,7 +394,7 @@
 	<p class="text-sm mb-2">The scan dialog asks for a profile name and offers a <strong>&ldquo;FCP/FCI check&rdquo;</strong> selector (the same three modes above, default <strong>Filetype + size</strong>) applied to every generated rule, plus an <strong>&ldquo;Ignore contents of files&rdquo;</strong> checklist (all checked by default except <code class="font-mono text-sm">EF.ARR</code>; the header checkbox checks or unchecks the whole list) of frequently-overwritten files whose contents are skipped: <code class="font-mono text-sm">EF.LOCI</code>, <code class="font-mono text-sm">EF.PSLOCI</code>, <code class="font-mono text-sm">EF.EPSLOCI</code>, <code class="font-mono text-sm">EF.5GS3GPPLOCI</code>, <code class="font-mono text-sm">EF.Keys</code>, <code class="font-mono text-sm">EF.KeysPS</code>, <code class="font-mono text-sm">EF.SMS</code>, <code class="font-mono text-sm">EF.Kc</code>, <code class="font-mono text-sm">EF.KcGPRS</code>, <code class="font-mono text-sm">EF.LOCIGPRS</code>, <code class="font-mono text-sm">EF.CBMID</code>, <code class="font-mono text-sm">EF.SMSS</code>, <code class="font-mono text-sm">EF.ACC</code>, <code class="font-mono text-sm">EF.EPSNSC</code>, <code class="font-mono text-sm">EF.START-HFN</code>, <code class="font-mono text-sm">EF.ARR</code>. Two further checked-by-default options <strong>&ldquo;Match first 4 bytes for&rdquo;</strong> <code class="font-mono text-sm">EF.IMSI</code> and <code class="font-mono text-sm">EF.ICCID</code> capture those files&rsquo; contents as a mask of only the first 4 bytes (uncheck for exact matching). A progress line shows <em>N / total files</em> with the current file path while scanning; during the scan the options are hidden and the buttons are locked. Rules are created only for files that actually exist on the card (a FCP template is returned); missing files are skipped. Custom files from the <strong>Custom files</strong> sub-tab are included under the same existence check.</p>
 
 		<h4 id="card-snapshots" class="font-medium mb-1">Card snapshots</h4>
-	<p class="text-sm mb-2">The list view has two tabs &mdash; <strong>Profiles</strong> and <strong>Card snapshots</strong>. A card snapshot is an immutable capture of the card filesystem: for every existing file it stores the path, symbolic name, file type, size (or record length/count), the raw FCI from the SELECT response, and the contents whenever the file is readable (no ignore list, no masking). The ICCID is decoded from EF.ICCID and shown next to the snapshot name.</p>
+	<p class="text-sm mb-2">The list view has two tabs &mdash; <strong>Profiles</strong> and <strong>Card snapshots</strong>. A card snapshot is an immutable capture of the card filesystem: for every existing file it stores the path, symbolic name, file type, size (or record length/count), the raw FCI from the SELECT response, and the contents whenever the file is readable (no ignore list, no masking). The ICCID is decoded from EF.ICCID and shown next to the snapshot name. The scan also measures each card command (SELECT, READ BINARY, READ RECORD) from command to response; the snapshot stores min/avg/max per command type and the total scan time, and the view shows these in the summary under the title plus the select/read times per file and the read time per record. Timings are informational only and are not used by checks or comparisons.</p>
 	<ul class="list-disc list-inside text-sm space-y-1 mb-3">
 		<li><strong>New snapshot</strong> &mdash; asks for a name and scans the card, then returns to the list.</li>
 		<li><strong>Import snapshot</strong> &mdash; loads a snapshot from a JSON file.</li>