Neo4j schema contract (schema.go) + conformance test + --emit schema

[lamwassi]

Is your feature request related to a problem? Please describe.

The Neo4j projection (parent issue) needs a single declared source of truth for its graph
vocabulary before any projector or writer can be held to it. Without it, the emitter and the
contract drift and --emit schema has nothing to serialize. This is milestone M1 of the
roadmap — the foundation every later milestone depends on.

Describe the solution you'd like

A new internal/neo4j/schema.go (sibling to internal/syntactic_analysis/v2emit/, off the
JSON hot path) that is the declarative contract:

• Node labels + merge keys + typed properties — :GoApplication, :GoModule, :GoType
  (kind property), :GoCallable, :GoField, :GoBodyNode, :GoExternal, and the neutral
  :Artifact/:Package/:ConfigKey. Shared :GoSymbol merge-label; :GoCanNode marker on
  every can:// node.
• Relationship types + endpoints — containment (GO_HAS_MODULE/GO_DECLARES/
  GO_HAS_METHOD/GO_HAS_FIELD/GO_HAS_BODY_NODE), GO_CALLS/GO_RESOLVES_TO,
  GO_EMBEDS/GO_SATISFIES, GO_IMPORTS, the neutral repo edges, and GO_PROVIDES/
  GO_UNRESOLVED_IMPORT.
• The reserved L3/L4 overlay vocabulary declared now (zero rows until the levels land):
  GO_HAS_BODY_NODE/GO_CFG_NEXT(_k=kind)/GO_CDG/GO_DDG(_k=var,prov)/GO_PARAM_IN/
  GO_PARAM_OUT/GO_SUMMARY + body-node statement kinds.
• _k relationship discriminants on GO_CFG_NEXT, GO_DDG, DECLARES_DEPENDENCY.
• Derived DDL — one uniqueness constraint per distinct (mergeLabel, key) (no second
  list to keep in sync) + curated indexes incl. the :GoCanNode id range index for prefix seeks.
• SCHEMA_VERSION = "2.0.0" + buildSchemaDocument(); --emit schema serializes it to
  schema.neo4j.json.
• A conformance-test scaffold (schema_test.go) that M3 fills: the real projector may
  never emit an undeclared label / relationship / property.

Full catalog + rationale: docs/design/Neo4j_projection/specs/neo4j-projection.md §3.

Gate (this PR must pass):

• --emit schema writes a schema.neo4j.json that parses and lists every declared family.
• Derived-DDL test: exactly one constraint per distinct (mergeLabel, key), no duplicates.
• go build + go vet clean; checked-in schema.neo4j.json regenerated and committed.

Describe alternatives you've considered

• Hand-written constraint list — rejected: derive constraints from the labels so a new label
  brings its own constraint and the two cannot drift.
• Defer the L3/L4 overlay vocabulary — rejected: declaring it now (empty) keeps the later
  L3/L4 train a pure projection-fill, not a graph-schema change.

Additional context

Parent: the Neo4j projection coordination issue under epic #94. Mirror the existing test idiom
(TestGateL1_*, TestDeterminism_* in v2emit) for the Neo4j gates.