fix(slab): correct HOST bitmapOff double-subtraction in buildLayoutV12_1

[0x-SquidSol]

Summary

• buildLayoutV12_1 computed HOST bitmapOff as V12_1_ENGINE_BITMAP_OFF - V12_1_ENGINE_OFF (1016 - 648 = 368)
• But V12_1_ENGINE_BITMAP_OFF (1016) is already relative to engineOff, consistent with computeSlabSize and all other layout builders
• The subtraction double-adjusted, producing 368 instead of 1016
• This caused wrong accountsOff for HOST test slabs and 8 failing drift-check tests
• One-character fix: remove the - V12_1_ENGINE_OFF subtraction

Test plan

• drift-check tests: 46 passed (was 38 passed + 8 failed — all 8 HOST bitmapOff failures resolved)
• Full suite: 737 passed (8 remaining failures are pre-existing ADL mock issues)

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: d11ca9cb-022f-4cbb-aebb-165aabf67709

📥 Commits

Reviewing files that changed from the base of the PR and between 2f80d79 and 29222b8.

📒 Files selected for processing (1)

• src/solana/slab.ts

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