vcon 0.9.2: emit vcon syntax param, drop empty defaults, add WTF analysis helper

[howethomas]

Summary

• build_new() now sets "vcon": "0.4.0" per draft-ietf-vcon-vcon-core-02 §4.1.1. The field is deprecated in the draft ("The extension mechanism replaces the need for schema versioning") but retained for parser compatibility — emitting it by default avoids surprises with strict consumers.
• build_new() no longer initializes empty "group": [] and "redacted": {}. The speckit reserves group; downstream consumers no longer need to strip these defaults before serializing. Both fields are still populated lazily by their setters.
• New helper add_wtf_transcription_analysis() — sibling of add_wtf_transcription_attachment that places a WTF transcription into analysis[] (type: "transcription", vendor, product, schema, encoding: "json", JSON-stringified body). Use the new helper when transcripts are derived analysis output; use the existing _attachment helper for the canonical attachments[] placement shown in the speckit example.

Compatibility note

Removing the empty group/redacted defaults is a shape change. Any consumer that does vcon_dict["group"] (subscript) on a fresh build_new() output will now KeyError. Library internals already use .get(). Audit downstream code that reads these fields directly.

Test plan

• Full test suite: 253 passed, 7 pre-existing skips (FFmpeg/video gates).
• Built vcon-0.9.2-py3-none-any.whl and vcon-0.9.2.tar.gz via poetry build.
• Smoke-tested wheel in clean venv: Vcon.build_new() emits "vcon": "0.4.0", omits group/redacted, add_wtf_transcription_analysis writes one analysis entry and registers the wtf_transcription extension.
• TestPyPI dry-run (run after merge, before PyPI publish)
• PyPI publish (after merge + tag)

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