feat: add nucleus engine crate

[bmuddha]

What changed

Added the nucleus crate with shared engine primitives, optional Heed integration, byte-size constants, and cooperative shutdown support.

Why

The stack needs a small common crate for engine-wide primitives that should not live inside account storage, ledger, or keeper.

Impact

• Introduces the shared Slot alias and MB/GB constants.
• Adds optional heed and shutdown features.
• Provides ShutdownManager, service handles, cancellation tokens, and termination reasons for engine services.

Reviewer notes

The shutdown manager owns one cancellation token and gives registered services a bounded window to report termination after OS shutdown, internal cancellation, or premature service exit.

Follow-up

ledger and keeper use the shutdown feature upstack.

[bmuddha]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat: adds top level engine crate for global orchestration #24
• feat: added internal built-in magic root program #23
• feat: implement transaction processor crate #22
• feat: implements keeper crate - state orchestrator #17
• feat: implement superblock based ledger crate #15
• feat: add optimized accountsdb crate #14
• feat: add nucleus engine crate #18 👈 (View in Graphite)
• feat: add svm crate #21
• feat: add program runtime crate #20
• feat: add transaction context crate #19
• feat: add customized solana-account crate #13
• master

This stack of pull requests is managed by Graphite. Learn more about stacking.

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