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.

frontend/tests/fs_probe.test.js

@@ -22,7 +22,7 @@ function extractFunc(src, name) {
 }
 
 let code = 'var pysimFsTreeRoot = null;\nvar _pysimFsProbe = null;\nvar pysimFsSort = "fid";\n';
-for (const fn of ['getParentSel', 'pysimFsSortChildren', 'pysimFsLoadChildren', 'pysimFsSelectBody', 'pysimFsProbeUi', 'pysimFsProbeAll']) {
+for (const fn of ['getParentSel', 'getParentPath', 'pysimFsSortChildren', 'pysimFsLoadChildren', 'pysimFsSelectBody', 'pysimFsProbeUi', 'pysimFsProbeAll']) {
 	code += extractFunc(html, fn) + '\n';
 }
 code += 'globalThis.esc = s => s;\nglobalThis.t = s => s;\nglobalThis.pysimCustomInject = () => {};\n';
@@ -123,6 +123,19 @@ test('stop halts the walk and still reports a summary', async () => {
 	assert.strictEqual(els['pysim-fs-probe-btn'].textContent, 'Probe all files');
 });
 
+test('select bodies carry the parent path and set allow_probe only for custom files', async () => {
+	root([df('DF.A', '5f01'), Object.assign(ef('EF.CUSTOM', '6fcc'), { custom: true })]);
+	const body = pysimFsSelectBody(pysimFsTreeRoot.children[0]);
+	assert.deepStrictEqual(body.parent_path, ['MF']);
+	assert.strictEqual(body.parent_sel, 'MF');
+	assert.ok(!body.allow_probe);
+	const custom = pysimFsSelectBody(pysimFsTreeRoot.children[1]);
+	assert.deepStrictEqual(custom.parent_path, ['MF']);
+	assert.strictEqual(custom.allow_probe, true);
+	const nested = { name: 'EF.1', fid: '6f01', isDir: false, parent: pysimFsTreeRoot.children[0] };
+	assert.deepStrictEqual(pysimFsSelectBody(nested).parent_path, ['MF', '5f01']);
+});
+
 test('children of an absent directory are neither fetched nor selected', async () => {
 	const stale = Object.assign(df('DF.C', '5f03'), {
 		exists: false,