Follow a record into its fields, name the metrics, name the version

Three things the first export pass got wrong for a real study.

**Dotted paths.** A node returns a record, not a scalar — the numbers arrive
inside `final_metrics` — so `--metrics final_metrics.train_loss` yielded an
empty column and `--metrics final_metrics` yielded the whole record in one
cell. Both sides of the wide table now take dotted paths, and the defaults
reach the same depth: every number a result carries is a column named by its
path, and inputs are compared leaf by leaf, so two configurations differing in
one field give that field as the axis rather than two blobs that are merely
not equal. Lists stay whole — a curve belongs in the long table.

**`--list`.** Metric names are flow-qualified, so `--name train_loss` matched
nothing and said only that. `fluksio export metrics --list` prints the names
the selection carries, and an empty export made with `--name` points at it.

**A version to compare.** The CLI ships ahead of the engine and a stale one
answered a flat 404 with nothing anywhere in the API to tell how old it was.
The engine reports `version` on `/observability/summary`, `fluksio status`
prints it, and a 404 from export now names both versions — or says "older"
when the field itself predates the engine. Bumped to 0.1.5, which is what
makes the number worth reading.

Also formats `flow/metrics.py`, which had been committed unformatted and was
the last `ruff format --check` failure.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A9Hdrmf2cwNABCnE5x9UJa

backend/fluksio/api/routes/observability.py

@@ -17,6 +17,7 @@ from sqlalchemy import ColumnElement, Integer, cast, func
 from sqlalchemy import select as sa_select
 from sqlmodel import col, select
 
+from fluksio import __version__
 from fluksio.api.deps import FlowControllerDep, SessionDep, get_current_user
 from fluksio.api.routes.runs import elapsed_ms
 from fluksio.core.config import settings
@@ -51,6 +52,11 @@ class HealthSummary(BaseModel):
     nodes: dict[str, int]
     queue: dict[str, Any]
     loop_lag: dict[str, float]
+    #: What this engine is running. A client ships ahead of the engine it
+    #: talks to — a `pip install -U` upgrades one and not the other — and a
+    #: route the client knows and the engine does not answers a flat 404. This
+    #: is what turns that into a sentence. Absent means older than this field.
+    version: str = ""
 
 
 class SeriesPoint(BaseModel):
@@ -220,6 +226,7 @@ async def read_summary(request: Request, controller: FlowControllerDep) -> Any:
             if watchdog is not None
             else {"ewma": 0.0, "max_60s": 0.0}
         ),
+        version=__version__,
     )