refactor: centralize deployment lifecycle

[lthibault]

Summary

• Add the new host ww::stem::Source boundary.
• Add a deployment lifecycle owner.
• Start host-local epoch sequencing at 0.
• Share finalized-depth Atom polling between bootstrap and follow operations.
• Revoke the active deployment before replacement while teardown and preparation run concurrently.
• Split deployment preparation from activation and complete teardown before the root swap.
• Keep static and no-Stem deployments on the common lifecycle path.
• Delete crates/stem, cell::epoch, EpochService, the Atom Finalizer, Provenance, and obsolete bootstrap machinery.

Independent review fixes

• Correct the capabilities documentation to reference ww::stem::atom::Source.
• Return a retryable, non-authoritative Atom snapshot error when the chain tip is below the configured confirmation depth.
• Add a regression test that verifies no block becomes authoritative before the configured depth is available.

Deferred work

• Phase 3b
• Phase 3-IPNS
• Routing / ARCH-13
• capnp/stem.capnp

Validation

• cargo fmt --all -- --check
• cargo check --workspace --all-targets
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace --no-fail-fast
• git diff --check

All listed validation commands passed.