fix(cli): copy chunk metadata before id assignment to break dict aliasing

[ww2283]

Fixes #7.

Problem

Chunkers can return chunks for one file that alias the same metadata dict object. Both LeannCLI._assign_chunk_ids and LeannCLI._assign_unique_chunk_ids wrote each per-chunk id into that shared dict via setdefault, so the last chunk's id overwrote all previous ones. LeannBuilder.add_text resolves the passage id from metadata["id"], so every chunk of a multi-chunk file got the last chunk's id — corrupting passages.idx and the faiss id map, and failing leann verify with "id appears with different text" on every multi-file incremental build.

Fix

Copy the metadata dict at id-assignment time in both assigners: c["metadata"] = {**c.get("metadata", {}), "id": sid}.

Verification

• New unit tests reproducing the shared-dict aliasing (tests/test_incremental_build.py::test_assign_unique_chunk_ids_with_shared_metadata_dict, ::test_assign_chunk_ids_with_shared_metadata_dict) — red on base, green with fix.
• E2E: two-leg incremental IVF build (--sync-key, 2 files then +3 multi-chunk files, 59 chunks each) — leann verify exits 0, all 295 passage ids distinct, zero duplicate-id lines.
• ruff check / ruff format --check clean; TDD exit gate PASS (0 self-inflicted test/lint findings).