refactor: switch custom predicate stdout to JSONL

[kurasaiteja]

What does this PR do?

• Switch custom predicate stdout protocol from a single JSON object ({"selectedCrates": [...]}) to JSON Lines ie. one tagged record per line
• Replace PredicateOutput struct in symposium-sdk with a streaming PredicateEmitter API
• Rename the wire field from "crate" to "name" inside selectedCrate records for consistency
• This enables adding future record types (e.g. watchedFile for cache invalidation) without breaking the schema
• unknown keys are skipped for, not rejected for forward compatibility as this is a breaking change to symposium-sdk

Wire format

Each non-blank stdout line is a JSON object with exactly one key:

{"selectedCrate": {"name": "serde", "version": "1.0.217"}}
{"selectedCrate": {"name": "tokio", "version": "1.40.0"}}

Disclosure questions

AI disclosure.

• The AI tool authored small parts of the code (e.g., autocomplete, comments)

Questions for reviewers.