v1.4.1 — diff_since_cache reconstruction is now hash-exact

Fixed

• diff_since_cache reconstruction is now offset-aware and usually hash-exact. Cached text is rebuilt by padding to each chunk's start_pos/end_pos (chunkers strip content but offsets span the raw region), with multiple hash-validated candidates and a least-churn fallback for legacy rows. A one-line edit in a multi-chunk file now diffs as exactly +1/-1 with diff_exact: true instead of phantom blank-line edits at old chunk boundaries.
• CodeChunker: final chunk and line-fallback chunks now set end_pos from the raw region length (consistent with mid-chunks), so stripped trailing whitespace — including the file's final newline — is recoverable from offsets. New chunk IDs apply only when a file is re-chunked; legacy rows degrade gracefully to best-effort diffs.
• merge_similar_chunks joins with the true inter-content distance from offsets ("\n" * gap) instead of a hardcoded "\n\n", keeping merged chunks reconstruction-faithful.