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.

docs/api.md

@@ -277,15 +277,22 @@ Read file content. Auto-detects transparent vs record files.
 
 Returns transparent data:
 ```json
-{"success": true, "sw": "9000", "file_type": "transparent", "data": "..."}
+{"success": true, "sw": "9000", "file_type": "transparent", "data": "...",
+ "apdu_times": [{"type": "select", "ms": 12}, {"type": "read_binary", "ms": 9}]}
 ```
 
 Returns records:
 ```json
 {"success": true, "sw": "9000", "file_type": "linear_fixed",
- "records": [{"num": 1, "data": "..."}, {"num": 2, "data": "..."}]}
+ "records": [{"num": 1, "data": "..."}, {"num": 2, "data": "..."}],
+ "apdu_times": [{"type": "select", "ms": 12},
+                {"type": "read_record", "ms": 11}, {"type": "read_record", "ms": 13}]}
 ```
 
+`apdu_times` reports each command's duration (command sent to response
+received) classified as `select`, `read_binary` or `read_record`; the PWA uses
+it for snapshot timing statistics. Other commands are not reported.
+
 ### `POST /api/write`
 
 Write raw hex data to a file.
@@ -316,7 +323,8 @@ Returns:
 ```json
 {"name": "EF.ICCID", "fid": "2FE2", "file_type": "transparent",
  "file_size": 10, "record_len": null, "num_of_rec": null,
- "fci_hex": "621082024021...", "exists": true}
+ "fci_hex": "621082024021...",
+ "apdu_times": [{"type": "select", "ms": 12}], "exists": true}
 ```
 
 `fci_hex` is the raw FCP template (`'62'`) from the SELECT response, used by