refactor(spec): move observability seam into the spec layer

[alexanderlhicks]

Summary

Completes the spec → node decoupling. A prior refactor relocated the
Interval type and protocol constants into the spec layer but left one
upward dependency behind: fork_choice.py and state_transition.py
still imported observability from node. This moves that last edge.

node/observability/ → spec/observability/. The package is purely the
vendor-neutral observer seam: a SpecObserver Protocol, a
_NullObserver no-op default, the observe_* context-manager timers, and
set_observer. The concrete Prometheus backend stays in node/metrics
and the startup set_observer(...) wiring stays in the CLI — only the seam
moves. The test moves to the mirrored tests/lean_spec/spec/observability/
path per the test-structure convention.

After this:

grep -rn "lean_spec.node" src/lean_spec/spec/   # empty

The spec layer no longer depends on node in either direction.

Why move the seam (alternative considered)

The seam is already a dependency-injection design: spec emits through a
Protocol, the default is a no-op so spec imports stay side-effect-free, and
clients register a real observer at startup. That seam is a spec-level
concern — the spec defines what it observes — so it belongs in the spec
layer with the no-op default. The alternative, leaving it in node and
threading an observer object through every spec entry point, was rejected:
it churns every state-transition signature and fights the existing
module-level singleton design.

Safety

Pure relocation, no behavior change — the observer classes and context
managers move verbatim; only import paths change.

Verification

Validated with uv run --no-sync against the installed dependency set:

• grep -rn "lean_spec.node" src/lean_spec/spec/ → empty.
• ruff check (full) + ruff format --check → clean.
• ty check → clean.
• codespell on changed files → clean.
• pytest tests/lean_spec/spec/observability tests/lean_spec/spec/forks/lstar → 243 passed.

Note: full just check / full pytest could not be run locally because
main bumped lean-multisig-py to v0.0.6, whose maturin/Rust build
fails in my local environment (unrelated to this change). CI should run the
complete suite against a working build.

🤖 Generated with Claude Code

[alexanderlhicks]

Just for context the two prior refactors were #819 and #817. I happened to basically have them done locally so this PR was tweaked at the last moment when pulling the latest changes from main.

Completes the spec → node decoupling. A prior refactor relocated the
Interval type and protocol constants into the spec layer but left one
upward dependency behind: fork_choice.py and state_transition.py
still imported observability from node. This moves that last edge.