fix: parent-scoped file selection; never mutate the pySim model

pySim's lchan.select() resolves names against global selectables (self +
parent chain + MF children + applications) and falls back to probe_file(),
which blindly SELECTs an unknown FID and permanently injects a dynamically
named DF.XXXX/EF.XXXX into the running filesystem model. Probing a whole tree
or scanning a snapshot with custom files therefore polluted the model, made
tree branches show children of the wrong object, and could persist phantom
files into snapshots.

- server: new _select_with_parent()/_select_path() walk the requested parent
  path (new parent_path field, parent_sel kept as legacy fallback) strictly
  through the model and call lchan.select_file() only; model-unknown 4-hex
  segments are probed only with allow_probe and the temporary child pySim
  adds is detached again via the cleanup callable that the four handlers
  (/api/tree|select|read|write) now run in a finally block
- frontend: getParentPath() builds the segment chain (MF, ADF names, FIDs)
  and all tree/select/read/write bodies plus the snapshot/profile walker send
  parent_path; allow_probe is set only for custom files; the blind retries
  in the file manager were dropped
- tests: tests/test_select_scope.py (duplicate-FID resolution, no APDU for
  unknown non-custom files, probe+detach, model unchanged); fs_load/fs_probe
  assertions for parent_path and allow_probe; docs/api.md and AGENTS.md
  document the contract; SW cache v117 -> v118.

docs/api.md

@@ -272,7 +272,7 @@ Returns the current STK session state.
 Read file content. Auto-detects transparent vs record files.
 
 ```json
-{"name": "EF.ICCID", "fid": "2FE2", "parent_sel": "3F00", "mode": "raw"}
+{"name": "EF.ICCID", "fid": "2FE2", "parent_path": ["MF"], "mode": "raw"}
 ```
 
 Returns transparent data:
@@ -298,12 +298,12 @@ it for snapshot timing statistics. Other commands are not reported.
 Write raw hex data to a file.
 
 ```json
-{"name": "EF.ICCID", "fid": "2FE2", "data": "A0A1A2...", "parent_sel": "3F00"}
+{"name": "EF.ICCID", "fid": "2FE2", "data": "A0A1A2...", "parent_path": ["MF"]}
 ```
 
 For record files:
 ```json
-{"name": "EF.ADN", "fid": "6F3A", "data": "A0A1...", "record_nr": 1, "parent_sel": "7F10"}
+{"name": "EF.ADN", "fid": "6F3A", "data": "A0A1...", "record_nr": 1, "parent_path": ["MF", "7F10"]}
 ```
 
 Returns:
@@ -316,9 +316,19 @@ Returns:
 Select a file by name or FID, with optional parent selection.
 
 ```json
-{"name": "EF.ICCID", "fid": "2FE2", "parent_sel": "3F00"}
+{"name": "EF.ICCID", "fid": "2FE2", "parent_path": ["MF"]}
 ```
 
+`parent_path` lists the path segments from MF to the parent (ADF names or
+FIDs); the legacy single-segment `parent_sel` is still accepted but is only
+unambiguous for ADFs. Resolution is strictly parent-scoped: model-known files
+are selected through the requested parent only (pySim `select_file()`), never
+via pySim's global selectables or its `probe_file()` model injection, so a
+same-FID file under another parent is never picked and the filesystem model
+is not modified. `allow_probe: true` (PWA custom files) additionally allows a
+model-unknown 4-hex FID to be selected directly; any temporary model object
+created for it is detached again before the response is sent.
+
 Returns:
 ```json
 {"name": "EF.ICCID", "fid": "2FE2", "file_type": "transparent",
@@ -340,6 +350,9 @@ Get directory listing with typed children.
 {"name": "MF", "fid": "3F00"}
 ```
 
+Use `parent_path` (or the legacy `parent_sel`) to list a subdirectory, e.g.
+`{"name": "DF.GSM-ACCESS", "fid": "5F3B", "parent_path": ["MF", "ADF.USIM"]}`.
+
 Returns:
 ```json
 {"exists": true, "name": "MF", "fid": "3F00", "file_type": "df", "children": [{"name": "EF.ICCID", "fid": "2fe2", "isDir": false}]}