Sessions: delete the production-dead TranscriptSource parse/persist pipeline

[ScriptedAlchemy]

Summary

Host ingest moved to observation admission (admit_*_observations, capture_*_snapshot_observations), but the pre-admission pipeline, where TranscriptSource::parse_new/try_parse_new produce ParsedTranscript rows and persist_parsed_transcript writes them, still exists. Tests are its only callers, so those tests cover an implementation that production no longer runs.

Evidence (origin/master 9b79b48)

• try_ingest_source / try_ingest_source_with_store (crates/tracedecay-sessions/src/runtime/source.rs) have 0 non-test callers. The only production call to try_parse_new is prepare_loaded_transcript, which only try_ingest_source_with_store reaches. persist_parsed_transcript is reached only through that chain as well.
• These parse_new impls are reached only through that chain: CursorEventSource, CursorSweepSource, CodexSource, KiroSource, ClineLikeSource, VibeSource. Production Cursor ingest uses admit_cursor_jsonl_observations. Kiro uses capture_kiro_snapshot_observations, which calls parse_snapshot directly.
• Cursor's parse_cursor_jsonl and event_dispatch_messages build tool_dispatch rows that duplicate canonical_cursor_compatibility_message_fields in tracedecay-store/src/canonical_projection.rs. Two renderers now produce one shape, and only the canonical one runs in production.
• KiroSource::parse_new swallows errors with .ok().flatten().
• About 75 test call sites depend on it: transcript_ingest_suite/{cursor,cursor_composer,codex,codex_ingest,codex_usage,codex_response_items,kiro,vibe,cline_like,restart_atomicity,source_identity}.rs, session_suite/lcm_raw.rs, dashboard_api_test/runtime.rs, tracedecay-sessions source/tests.rs and hosts/{codex,cursor}/tests.rs, and benches/transcript_ingest.rs.

Cutover

Remove parse_new/try_parse_new from TranscriptSource along with try_ingest_source*, prepare_loaded_transcript, persist_parsed_transcript, and the host parse bodies that become dead. Move each test that asserts real host-format behavior onto the production admission entry points, and delete the tests that only protect the parse pipeline.

Found by the legacy-api-sweep sessions lane.

[ScriptedAlchemy]

Delivered by #2748 (squash 2764eaa). The parse/persist driver, every host parse_new/try_parse_new, persist_parsed_transcript, and the duplicate Cursor tool_dispatch renderer are deleted; canonical_*_compatibility_message_fields remain. Behavior-bearing tests now drive observation admission through the production project/user passes and host captures. Gaps this exposed: #2742, #2745, #2746.