refactor(#2949): the build-log row carries its node id

`QueueInner` was `{ sched, node_rt }`, where `node_rt` held exactly one
datum per node: the `build_logs` row id. It existed because a `hive_jobq`
node payload is immutable after insert while the log row is created when
the build starts — so the link could not ride the node.

Invert it: the log row names its node (`build_logs.node_id`, one migration
in the existing `schema_versions` framework). Same single-home property,
in the direction the type system allows.

`QueueInner` is now just the scheduler. That is the point: the queue holds
no per-node side map, so nothing has to be locked alongside the graph.

Deleted as a consequence, each surfaced by dead-code analysis after the
edit above rather than predicted:

- `NodeRuntime`, `node_rt`, `set_build_log_id`, and `build_log_id_of`
  (which linear-scanned the map to match a wire `u64` against opaque
  `NodeId`s). The lookup is an indexed query now.
- `struct Ctx`, entirely. It carried `coord` + `dag_id` + `node_id` into
  the executors so the build-log callback could reach the queue; without
  the callback, `coord`/`dag_id` were never read and `node_id` was already
  on the `Claim` both executors receive.
- `QueueInner::node_running`, which existed only for `set_build_log_id`'s
  "only while running" guard.
- The `Fn(i64)` callbacks on `prebuild_toplevel` / `swap_update` /
  `priv_run_inner`, replaced by a `node_id: Option<u64>` passed down. The
  id travels one way now instead of being registered back.

`meta.rs`'s `nix_logged` passes `None` deliberately: its callers reach it
from outside the queue as well as inside, and nothing reads the link for
them yet.

`id_for_node` takes `MAX(id)` rather than assuming uniqueness — a retried
node opens a second row and the panel wants the current attempt. The test
moved to where the behaviour lives and covers that, plus survival across
completion and non-collision with node-less rows.

hive-c0re/src/meta.rs

@@ -1590,7 +1590,12 @@ async fn nix_logged(dir: &Path, args: &[&str], agent: &str, kind: &str) -> Resul
     let cmdline = format!("nix {}", nix_argv(args).join(" "));
     let logs = crate::build_logs::global();
     let log_id = logs.as_ref().and_then(|h| {
-        h.start(agent, kind, &cmdline)
+        // No node id: `nix_logged`'s two callers are meta-flake operations
+        // reached from outside the queue as well as from inside it, and the
+        // agent+kind+time listing is how they're surfaced today. Linking them
+        // to a node would mean threading the id through `meta`'s public API
+        // for no current reader — worth doing when something wants it.
+        h.start(agent, kind, &cmdline, None)
             .map_err(|e| {
                 tracing::warn!(error = ?e, %kind, "build_logs: start failed (meta log dropped)");
             })