SDK: GoAnalysisBackend, Neo4j projection #426

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

Epic #94 brings codeanalyzer-go to canonical schema v2 "in both projections
(analysis.json + Neo4j)". The JSON projection (L1 + L2) is in flight; the Neo4j
projection is not yet built, and the python-sdk has no go analysis package, so a graph
database holding a Go CPG cannot be produced or queried. This issue coordinates the Neo4j
half of #94 across the two repos.

Describe the solution you'd like

A pure, identity-preserving second projection of the frozen v2 envelope into a labeled
property graph — keyed on the identical can:// ids so JSON and graph are interchangeable —
plus the SDK consumer that reads it. Vocabulary is GO_/Go-namespaced for a polyglot
database, with the neutral :Artifact/:Package/:ConfigKey layer reused verbatim.

Spec (full design + decisions — do not paste, link):
codeanalyzer-go/docs/design/Neo4j_projection/specs/neo4j-projection.md
Roadmap (milestones, commit boundaries, gates):
codeanalyzer-go/docs/design/Neo4j_projection/roadmap.md

Children (filed just-in-time on their repos, attached as sub-issues of this one):

• M1–M6 analyzer --emit neo4j (codeanalyzer-go) — internal/neo4j/
  {schema, rows, project, cypher, bolt, emit}.go + CLI wiring. May be one issue with the
  milestones as a checklist, or a sub-stack per PR if reviewers prefer finer PRs.
• M7 GoAnalysisBackend — type-centric
  Go facade + Go models + GoBackend union; pins the analyzer version only after the
  analyzer release is cut (lockstep).
• Docs + CHANGELOG (finishing-cldk-work).

Locked design decisions (full table in the spec):
N="Go"/P="GO_"; one :GoType + kind property; GO_EMBEDS vs GO_SATISFIES;
flattened source_file/error_channel/is_goroutine/is_deferred; narrower label set (no
decorator/attribute/variable); neutral repo layer reused verbatim; full L3/L4 overlay
vocabulary declared now, zero rows until the levels land; type-centric SDK facade.

Describe alternatives you've considered

• Block the projection on L3/L4 first — rejected: --emit neo4j ships now at full-depth =
  max implemented (L2) and grows additively; the reserved overlay vocabulary means L3/L4 fill
  it with no schema change.
• Per-kind type labels (like TS) — rejected: re-introduces at the graph layer the per-kind
  split the JSON schema deliberately collapsed into one kind field; breaks projection symmetry.
• Class-centric SDK facade (reuse get_classes) — rejected: Go has no class; the facade is
  type-centric (get_types/get_functions/get_methods_of), the SDK's first class-less surface.

Additional context

Scope guard: the analyzer is a pure graph provider of the SDG substrate — no
taint_flows; labeled reachability/slicing/source-sink are SDK Cypher queries, a later
train. Release plan (per #94): "Neo4j projection and the JSON schema move in lockstep."