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.

🤖 Generated with Claude Code

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
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Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?

[BigVan]

Hello~ actually LSMTFile supports unlimited layer size via

overlaybd/src/overlaybd/lsmt/file.cpp

Line 1635 in 222aa97

p = copy_lsmt_index(file, job->ht);

You can use open_files_ro() to open multiple layers as a single IFile*. Use this IFile as another ReadOnly layer, like Russian nesting doll :D