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/cli-reference.md

@@ -67,7 +67,7 @@ aliases:
     graph: <name>
     branch: <name>
     format: <output-format>
-queries:                          # top-level stored-query registry (single-graph mode); mirrors top-level `policy`
+queries:                          # top-level registry — applies only to a bare-URI (anonymous) graph; a graph served by name uses its `graphs.<id>.queries`. Mirrors top-level `policy`.
   <query-name>: { file: <path-to-.gq> }   # mcp.expose defaults to true
 policy:
   file: ./policy.yaml