feat(neo4j): derive code onto declarations, matching python and java

[rahlk]

Replaces #136 (same commit, cherry-picked onto main after the stack below it merged).

The gap

TypeScript source text was unreachable from the graph: :TSCallable had no code, and
:TSModule had no source to slice from — nodes carried start_line/end_line, not the
span.bytes the tree uses. Neither route worked. python and java hand a consumer code directly.

The JSON model was never the disagreement

	JSON	Neo4j code
python	module source once + span.bytes; no per-callable code	derived at projection
typescript	module source once + span.bytes; no per-callable code	was missing
java	code stored per callable	carried through

python and TypeScript already agree on the JSON model. python then completes the contract on the
graph side (neo4j/project.py::_span_code):

"A declaration's text: the owning module's source sliced by the node's utf-8 byte span. Schema
v2 stores source once per module, so the graph's code property … is derived here at projection
time (#104)."

TypeScript stopped short of that step. This adds it — no JSON change, no text stored twice.

Details matched deliberately

• spanCode slices a Buffer, not the string. span.bytes are UTF-8 byte offsets and JS
  strings are UTF-16, so source.slice() would cut multi-byte characters.
• ts_code_fts, a fulltext index over TSCallable.code, mirroring python's py_code_fts.
  Text on a node that cannot be searched is half a feature.
• code is declared on every label that receives it — TSCallable plus the five type labels,
  since typeProps is shared. python declares it only on PyClass/PyCallable; declaring it where
  it is actually emitted keeps the conformance gate meaningful.

Verification

Live graph, sample-app: 41 of 42 callables carry code (the holdout is a bodiless
declaration). Sample value:

"Get" → "function Get(path: string): MethodDecorator {\n  return () =>…"

WHERE c.code CONTAINS "..." now works. Suite: 240 pass, 0 fail; gen:schema clean.

Related

• java's JSON stores code per callable rather than slicing —
  analysis.json stores declaration text per callable; python and typescript store it once per module codeanalyzer-java#215, filed as a decision rather than a defect.
• docstring is still divergent: python and java carry it, TypeScript's JSON has comments on
  callables. That needs a shape decision, not a straight port, so it is not in this PR.