EF resolver: resolve by (DF, FID); split EF.ECC and EF.AD per DF

The decoder/encoder lookup now takes an optional DF context and resolves
DF-first, then name, then FID (the spec keys files by FID within their
DF; the name is a display label).  efDfFromPath derives the parent DF
segment of a canonical path.

EF.ECC is split into its two DF-specific decoders: DF.GSM (7F20)
transparent 3n (existing) and ADF.USIM linear-fixed X+4 - the new
efDecEccUsim reads the 3-byte Emergency Call Code, the Annex A alpha and
the Emergency Service Category (decoded per TS 24.008 Table 10.5.135d).
EF.AD is registered as DF-keyed entries (7F20 / ADF.USIM / ADF.ISIM),
reusing the existing encoder.

The df is threaded through the file-manager decoded-view/edit/save and
the snapshot/compare decode.  No ECC editor yet (read-only).

Frontend 1081 + Python 1090 tests green.

frontend/tests/fs_edit.test.js

@@ -29,7 +29,8 @@ let code = 'var pysimFsDecodedMode = false;\n'
 	+ 'var pysimFsSelected = null;\n'
 	+ 'var pysimFsTreeRoot = null;\n';
 for (const fn of ['pysimFsSetMode', 'pysimFsPillsEnabled', 'pysimFsEdit',
-	'pysimFsCancel', 'pysimFsResetEdit', 'pysimFsFindNode', 'pysimFsHasDecoder',
+	'pysimFsCancel', 'pysimFsResetEdit', 'pysimFsFindNode', 'pysimFsNodePath',
+	'efDfFromPath', 'pysimFsHasDecoder',
 	'pysimFsSyncContentVisibility']) {
 	code += extractFunc(html, fn) + '\n';
 }