ctor debug state setup via shared atomic in core

[lmmx]

This PR adds a shared AtomicBool (in syncdoc-core/src/debug.rs) and refactors the migrate crate to re-export the core crate's debug macro rather than provide a duplicate.

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syncdoc-core has:

• tests/: activated in tests/helpers.rs
• src/tests/: activated in src/lib.rs

syncdoc-migrate has:

• tests/: activated in tests/helpers.rs
• src/tests/: activated in src/tests/mod.rs

syncdoc has:

• cli_integration.rs (single module): activated in it
• roundtrip/: activated in roundtrip/helpers.rs