feat: add nucleus engine crate

[bmuddha]

What changed

Added the nucleus crate with feature-gated primitives shared across the engine stack.

Why

Ledger, runtime, storage, and orchestration crates need common types and lifecycle
coordination without assigning storage or execution policy to the shared dependency.

Closes #28.

Impact

• Introduces shared slot and byte-size types, ledger positions, runtime messages,
  metrics helpers, thread-local runtime state, and test support.
• Keeps Heed, ledger, metrics, runtime, shutdown, testkit, and TLS support behind
  narrow crate features.
• Provides ShutdownManager, service handles, ordered cancellation tiers, and
  bounded termination reporting.

Reviewer notes

Shutdown cancels the replication client, pacemaker, sequencer, and remaining
services in order. Each tier receives its own bounded window to report completion
before shutdown advances.

Follow-up

Ledger, keeper, processor, engine, and replication services consume these
feature-gated primitives 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: add state replicator crate on top of the engine #25
• 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: forked transaction-view for increased limits #33
• feat: add customized solana-account crate #13
• chore: import upstream Solana runtime crates #31
• master

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

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