async def is not surfaced — kind is "function" for async callables

[rahlk]

Problem

async def is not represented anywhere in the emitted schema. git grep -i "is_async"
on main returns nothing under codeanalyzer/schema/ or codeanalyzer/syntactic_analysis/.
The builder does match ast.AsyncFunctionDef — but only to find callables, never to
record what it found; PyCallable.kind is set to the literal "function" either way.

Verified on main (6f02581), fixture method async def fetch(self):

method 'fetch' kind=function decorators=["trace('method')"]

Nothing distinguishes it from a sync method. The only way to recover async-ness is to
slice module.source[span.bytes] and re-parse the text the analyzer already parsed.

This also hides await sites, async generators, and async with / async for from
anyone reading the tree rather than the source.

Scope boundary

Adds an async marker to callables. Does not model await as a distinct CFG or DDG
node kind, does not add async-aware dataflow, and does not touch the decorator issues
(#127, #128, #129).

Goals

• PyCallable carries an async marker, available from L1
• Set for async def at every depth: module-level, method, nested closure
• Async generators (async def containing yield) are not misreported
• Projected into Neo4j as a property on :PyCallable
• Bump Neo4j SCHEMA_VERSION (additive)

Caveats and known risks

• Schema v2 change — enters at designing-cldk-changes. Additive and defaulted, so
  monotonicity and existing consumers are unaffected.
• kind vs a boolean is the decision to settle. kind is already a discriminant
  ("function", "class", "module", "external"); adding "async_function" to it
  would break every consumer switching on kind, whereas a separate boolean leaves
  kind alone. Pick one deliberately — do not do both.
• Sibling analyzers. TypeScript has async too. The field name should be the
  canonical cross-language one, decided once here rather than renamed later.
• @staticmethod / @classmethod / @property are in the same situation — recoverable
  only by string-matching decorators. Deliberately out of scope, but the same decision
  shape; worth noting so it is not re-derived later.

Definition of done

• The fixture emits the async marker set on Account.fetch and unset on Account.helper
• A module-level async def and a nested async def closure both carry it
• An async generator carries it and is not confused with a plain generator
• analysis.json(-a 1) ⊆ ... ⊆ (-a 4) still holds
• Neo4j exposes the property and a query can select async callables without touching source