fix(claude): drop empty streaming-seed assistant lines on projection

[bdelanghe]

Problem

Resuming a projected Claude session works, but the next turn dies with:

API Error: 400 messages: text content blocks must be non-empty

Root cause

Claude Code streams an assistant message as several JSONL lines — the first an empty "seed" (text: "") superseded by the real-text line, both sharing one message.id. The Claude projector wrote that seed back as content:[{"type":"text","text":""}]. That message is API-invalid: on the next turn of a resumed session Claude replays the whole transcript to Anthropic, which rejects the empty text block. The bug was in build_assistant_content's text-only fast path, which (unlike the tool/thinking path) had no empty-text guard.

Fix

The projector now skips content-empty assistant turns (no text/thinking/tool-uses/delegations/file-mutations, and no attached tool-result events) and re-links the following turn's parentUuid to the dropped seed's parent — reusing the existing parent_rewrites machinery, so the uuid chain stays intact. The group's token total is re-expanded onto the surviving line as before.

Verification

• New TDD test test_projector_skips_empty_text_seed_and_relinks_chain (failed before, passes after).
• Full toolpath-claude suite: 230 passed, 0 failed. Clippy clean.
• Real end-to-end: re-exported a real captured session — empty text blocks 1 → 0; the seed step is gone and the surviving assistant line re-parents to the user turn.
• Updated the older test_projector_reexpands_group_usage_onto_every_line test that had encoded the old emit-empty-line behavior.

Version: toolpath-claude 0.12.0 → 0.12.1 (Cargo.toml, workspace dep, crates.json, CHANGELOG).

Versioning

toolpath-claude 0.12.1 → 0.12.2 — 0.12.1 was taken by main's ClaudeConvo::session_chain bump (#133), so the empty-seed fix gets its own patch.

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Stack (review bottom-up): #141 empty-seed projection fix → #142 opencode test isolation → #145 path resume --remote.

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[bdelanghe]

Good call — extracted it to Turn::is_content_empty() on toolpath-convo (the intrinsic "no text/thinking/tool-uses/delegations/file-mutations" predicate), and left the projector-specific tool-result-events check at the call site:

let is_droppable_seed = turn.is_content_empty()
    && !tool_result_events_by_parent.contains_key(&turn.id);

toolpath-convo 0.11.1 → 0.12.0 (additive), with a unit test for the new method. Pushed in 7ddfa5b.