Resolve graph config by identity, not server mode

Which policy/queries block applies for a graph was decided three different,
mode-dependent ways: single-mode boot used top-level even for a named graph;
multi-mode used per-graph (and silently ignored a top-level queries block); the
CLI used per-graph for a named target. So `queries validate --target prod`
could check a different registry than the single-mode server loaded, and a
named graph's per-graph policy/queries were silently shadowed.

Make config a function of graph IDENTITY: a graph served by NAME
(--target/server.graph, a graphs: entry) uses its own graphs.<name>.{policy,
queries}; a bare URI is anonymous and uses top-level. One rule, applied by
single-mode boot, multi-mode boot, and the CLI — so they can't diverge and the
CLI predicts the server exactly.

No silent ignore: serving a named graph while a top-level policy/queries block
is populated now refuses boot, naming the block (the multi-mode top-level-policy
bail, extended to queries and to single-mode-named). The CLI's `queries
validate` derives the schema URI and the registry from ONE selection, and a
positional URI forces anonymous (ignoring cli.graph) so the two can't come from
different graphs.

BREAKING (released behavior): single mode by name (--target/server.graph) with
top-level policy/queries previously used top-level; it now uses the per-graph
block and refuses boot if top-level is also populated. Bare-URI single mode is
unchanged. Loud, with migration text pointing at graphs.<name>.

- config: resolve_policy_file_for (policy sibling of query_entries_for, no
  top-level fallback) + populated_top_level_blocks for the coherence check.
- characterization tests (single-mode named -> per-graph; named + top-level ->
  bail; multi-mode top-level queries -> bail; CLI positional-URI -> top-level).
- docs: policy.md, server.md, cli-reference.md.

docs/user/server.md

@@ -6,7 +6,9 @@ Axum 0.8 + tokio + utoipa-generated OpenAPI. **Two modes** (v0.6.0+): single-gra
 
 ### Single-graph mode (legacy)
 
-`omnigraph-server <URI>` or `omnigraph-server --target <name> --config omnigraph.yaml`. Routes are flat — `/snapshot`, `/read`, `/branches`, etc. Behavior unchanged from v0.6.0.
+`omnigraph-server <URI>` or `omnigraph-server --target <name> --config omnigraph.yaml`. Routes are flat — `/snapshot`, `/read`, `/branches`, etc.
+
+**Config follows graph identity.** A bare `<URI>` is an *anonymous* graph and uses the **top-level** `policy.file` / `queries:`. A graph chosen by **name** (`--target` / `server.graph`) uses its own `graphs.<name>.{policy.file, queries}` — the same block multi-graph mode uses. ⚠️ *Changed from v0.6.0, which always used top-level config in single mode: a named-graph config that puts `policy`/`queries` at top-level now **refuses boot** and points you at `graphs.<name>.…` (move the block there). Bare-`<URI>` single mode is unchanged.*
 
 ### Multi-graph mode (v0.6.0+)