Key a node on the code it reaches, not on the repository around it

`sync` follows each node function's imports through the project's own modules
— stopping at the standard library, at anything installed, and at Fluksio
itself, whose checkout would otherwise be most of every digest — and records
the file list with what it hashed to. The engine hashes those files again when
the run is claimed, so the fingerprint is live rather than a snapshot, and
falls back to what sync recorded when it cannot see them: a remote worker's
runs used to share one empty digest, and therefore one key.

Three things follow. Editing a helper a node calls into re-runs that node, as
before. Editing something the node never reaches no longer re-runs anything —
a notebook two directories away was invalidating every arm. And
`Run.code_digest` is now the hash of its nodes' digests, so it is neither
looser nor tighter than "the code behind these numbers", which is what makes
it worth joining an exported table on.

`sync` says so too: it compares the per-node digest against the stored one, so
a helper edit prints `train: updated (flow, fit)` instead of `unchanged`. The
digest is read when the document is built rather than when the flow is
declared, so a second `sync()` in one process sees an edit between them.

Also: `fluksio runs` shows only the inputs that differ from what the flow
declares, fitted to the terminal, so a flow taking a few kB of json no longer
wraps every line.

Every existing cache entry misses once — the fingerprint changed shape.

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

backend/fluksio/flow/schemas.py

@@ -206,6 +206,23 @@ class NodeDef(BaseModel):
             "workers, which is right for everything that is not compute-heavy."
         ),
     )
+    code_files: dict[str, str] = Field(
+        default_factory=dict,
+        description=(
+            "The project modules this node's function reaches, dotted name to "
+            "the file on the machine that synced it. Written by `fluksio "
+            "sync`, which is the only side that can import the code and see "
+            "what it imports. Empty for a node drawn on the canvas."
+        ),
+    )
+    code_digest: str = Field(
+        default="",
+        description=(
+            "What those modules hashed to at sync. The engine hashes them "
+            "again when a run is claimed, and falls back to this when it "
+            "cannot see the files — a worker on another machine."
+        ),
+    )
 
     @field_validator("id")
     @classmethod