Rename the Neo4j PyCFGNode label to PyBodyNode

[rahlk]

Problem

The Neo4j label PyCFGNode names a control-flow concept, but it holds every entry of a
callable's body{} at every level: call nodes from L1, statement/return/branch/
entry/exit from L3, and formal_in/formal_out/actual_in/actual_out from L4.
Most of those are not CFG vertices. A nested call is explicitly a dataflow satellite that
never joins the spine — .claude/SCHEMA_DECISIONS.md records that decision — and param
vertices are not control flow at all. Since #120 the same node also carries call-site
detail, so the label is now doubly inaccurate.

It is also a projection divergence: analysis.json calls the container body{} while the
graph calls its contents CFG nodes, so the two projections use different words for one
thing.

Scope boundary

Renames the node label and its containment edge. Does not rename the genuinely
control-flow relationships (PY_CFG_NEXT, PY_CDG, PY_DDG) or the cfg/cdg/ddg
edge lists on the callable — those are correctly named. Does not change any emitted fact.

Goals

• PyCFGNode → PyBodyNode (node label and merge label)
• PY_HAS_CFG_NODE → PY_HAS_BODY_NODE
• Endpoint labels updated on PY_CFG_NEXT, PY_CDG, PY_DDG, PY_PARAM_IN, PY_PARAM_OUT, PY_SUMMARY, PY_RESOLVES_TO
• _cfg_ref → _body_ref in neo4j/project.py, so the code stops saying CFG where it means body
• schema.neo4j.json regenerated; README.md, CLAUDE.md and .claude/SCHEMA_DECISIONS.md updated
• codeanalyzer-typescript already uses TSBodyNode / TS_HAS_BODY_NODE; this adopts that name rather than coining a third

Caveats and known risks

• Breaking graph change: nodes in an existing database strand under the old label, and the
  pycfgnode_id uniqueness constraint renames with it. Needs a stated migration, not just
  a version bump.
• docs/handoff/schema.neo4j.json must NOT be regenerated — it is a frozen bundle pinned
  to codeanalyzer-python==1.0.1.
• CHANGELOG.md entries describing past releases must NOT be rewritten; add a new entry.
• Risk of confusion with "body" meaning source text (get_method_body returns a span
  slice). Judged acceptable — body{} is already the JSON field name.

Definition of done

• No PyCFGNode or PY_HAS_CFG_NODE in codeanalyzer/, test/, README.md, CLAUDE.md
  or the root schema.neo4j.json
• --emit neo4j produces :PyBodyNode merged on the same global ordinal id as before
• PY_CFG_NEXT and friends still connect body nodes, with only the endpoint label changed
• Full suite green