IEC 61850 file-service golden parity: Directory/Open/Read/Close/Delete #96

Goal

Implement capture-backed MMS File Services parity for the IED Simulator and reusable ARStack core, using the real external IEC 61850 client Open / Download / Delete traces as the wire oracle.

Golden wire profile

• FileDirectory [77]: root request keeps fileSpecification present as A0 02 19 00.
• FileOpen [72]: rooted-backslash file name + initialPosition=0, returns per-association FRSM handle.
• FileRead [73]: one MMS response may be COTP-segmented across multiple TPKTs; moreFollows is MMS-level, not TCP/TPKT-level.
• FileClose [74]: primitive FRSM request, empty successful response.
• FileDelete [76]: constructed request carrying the rooted-backslash GraphicString directly; successful response is primitive empty [76] (9F 4C 00).
• external IEC 61850 client groups physical COMTRADE companions client-side. Delete sends one FileDelete per physical path, then immediately refreshes FileDirectory.

Architecture

F0 — protocol/core

• Add service 76 to vector and span service enums/mappings.
• Add portable FileDelete request/response codecs and explicit destructive runtime API.
• Preserve the portable file layer as filesystem/socket/thread free.

F1 — server extension

• Add an external confirmed-service hook to the bounded static dispatcher/runtime rather than embedding host filesystem code in portable MMS core.
• Preserve existing negotiated MMS size handling and COTP response segmentation.

F2 — host simulator file backend

• Sandboxed --file-root.
• Per-association bounded FRSM table/cursors.
• Sorted bounded FileDirectory with size/timestamp.
• FileOpen/FileRead/FileClose.
• FileDelete only when explicitly enabled (--allow-file-delete).
• Reject traversal, drive/scheme ambiguity, symlink escape, invalid/missing handles and unsafe deletion.
• Shared root synchronization across simultaneous associations.

F3 — product wiring

• Simulator helper passes its managed file root.
• Files workspace can explicitly delete a physical remote file and refresh directory after success.
• Upload/rename remain out of scope.

F4 — regression / acceptance

• Exact BER regression for [72,73,74,76,77] including external IEC 61850 client golden shapes.
• Integration: Directory → Open → Read → Close.
• Integration: Delete physical companions → Directory refresh.
• Large FileRead proves one MMS response can be segmented by COTP.
• Multi-association handle isolation + path/symlink safety tests.
• Final acceptance requires a real external IEC 61850 client retest; CI/simulator evidence alone is not relabelled as third-party acceptance.

Keep PR #82 OPEN + DRAFT while this work is under external external IEC 61850 client acceptance.