Support signed MMS FRSM IDs during file transfer

[masarray]

Field evidence

Directory discovery is now correct, but Download selected still fails on a BCU endpoint after FileOpen/FileRead begins. The only surfaced first-chance exception is InvalidDataException.

Root cause addressed

ISO 9506 defines the FileOpen response frsmID as Integer32, not an unsigned value. The existing decoder rejected negative values and the FileRead/FileClose request builders also threw for any FRSM below zero. Several MMS servers allocate negative FRSM handles, so a valid FileOpen response was being converted into a failed transfer.

Changes

• accept the complete signed Integer32 range returned as FRSM
• echo the exact signed FRSM unchanged in FileRead and FileClose
• replace the -1 sentinel with a nullable FRSM in the interoperable transfer path
• add bundle download routing that preserves existing size limits, temporary-file cleanup, cancellation and progress behavior
• include exact request/response hex in failed bundle diagnostics

Regression coverage

• FileOpen response with FRSM -17 succeeds
• FileRead and FileClose encode -1, -17, int.MinValue, and positive IDs correctly

The existing public transfer path is left intact; ARSAS opts into the compatibility path for field interoperability.

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