feat(cypher): scalar + entity-introspection functions (suite tier 1)

Add single-argument functions to RETURN/WITH projections, dispatched via a
generic IDENT-call recognizer so new functions no longer need a dedicated lexer
keyword:

- labels(n)      -> ["<label>"]
- type(r)        -> relationship type
- id(n)          -> node/edge identity
- keys(n)        -> JSON list of the node's non-null property keys
- properties(n)  -> the node/edge properties JSON object
- toInteger/toFloat/toBoolean(x) -> numeric/boolean casts (null on non-numeric)

Previously labels()/id()/keys()/etc. fell into the unknown-function path and
silently projected empty (labels(n) returned ""); they now evaluate correctly.
Regex =~ in WHERE was already supported. Unit tests for each function plus
smoke-test coverage via the live query_graph CLI.

Refs: full Cypher read suite (tier 1 of 3)