Add RawContainerReader: read the SPBXDS container without assuming OWON's JSON schema

[alexlegoshin]

Problem

BinfileReader assumes every .bin file's JSON metadata follows the OWON/Hanmatek
schema (CHANNEL/DISPLAY/Current_Ratio/Current_Rate, etc.). Files from other
scopes built on the same OEM platform can use the same SPBXDS container but a
different JSON layout — for example, an AKIP-4122/7 capture I ran into uses a
lowercase channel array with Display_Switch/Reference_Zero/Voltage_Rate
fields instead, and its JSON has a trailing comma before the array's closing
bracket. Both of those make BinfileReader.ReadAsync throw:

System.Text.Json.JsonException: The JSON array contains a trailing comma at the end
which is not supported in this mode. Change the reader options. Path: $.channel |
LineNumber: 0 | BytePositionInLine: 686.
   at OwonBinfileReader.BinfileReader.ReadAsync(Stream stream, CancellationToken cancellationToken)

So today there's no way to open such a file with this library at all, even though the
container itself is fully readable.

What this PR adds

A new, independent RawContainerReader that parses only the vendor-agnostic parts of
the SPBXDS container — magic header, JSON length, the JSON text itself (returned
as-is, no assumptions about its schema or strictness), one Int16[] per data segment,
and the optional trailing INFO block:

using OwonBinfileReader.Raw;

var raw = await new RawContainerReader().ReadAsync(@"/path/to/file.bin");
// raw.RawJson    -> the metadata JSON, exactly as stored
// raw.Segments   -> one raw Int16[] per data segment, in file order
// raw.Tail       -> the optional trailing INFO block, if present

This lets callers handle files with a metadata schema BinfileReader doesn't
recognize by parsing RawJson themselves, instead of getting an exception with no
workaround.

BinfileReader itself is completely untouched — this is a purely additive,
opt-in alternative for the "I have a file this library can't parse" case, not a
replacement.

Testing

• 6 new tests in RawContainerReaderTests.cs: the AKIP-4122/7 fixture (single
  segment, no tail), the existing OWON fixture, a synthetic multi-segment +
  INFO-tail case (to exercise segment-boundary/tail detection that no single real
  fixture covers), and the three existing error paths (invalid header, invalid JSON
  size, stream not at position 0).
• Added testfiles/akip.bin, a real capture from an AKIP-4122/7 (firmware V7.6.0),
  as a small (~20 KB) fixture.
• All 27 tests (21 existing + 6 new) pass.

Docs

Added a short new README section ("Reading files from other scopes on the same
platform") documenting RawContainerReader, its motivation, and a usage example, right
before the existing TestApp section. No existing README content was changed.