Extend layer limit from 255 to 65535 by using 16-bit tag field

[xinyangge]

The SegmentMapping struct's tag field was 8-bit, limiting overlaybd to 255 stacked layers. This change extends the tag to 16-bit by borrowing 8 bits from moffset (55->47 bits), which still allows 64 PB per layer file - far exceeding practical needs.

Changes:

• index.h: Change tag from uint8_t to uint16_t, moffset from 55 to 47 bits
• file.h: Update MAX_STACK_LAYERS from 255 to 65535
• file.cpp: Update m_rw_tag, min_tag, warp_file_tag types to uint16_t
• index.cpp: Update ro_index_count parameter and merge limit to 65535
• config.h: Update MAX_LAYER_CNT from 256 to 65536

Note: This is a breaking change - layers created with this version are not compatible with older overlaybd versions.

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• Does the affected code have corresponding tests, e.g. unit test, E2E test?
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