fix(extraction): index const-bound functions inside a body as symbols

[danusha2345]

Fixes #1669.

Problem

const handleClear = () => {…} inside a component — every React handler written without useCallback — was never a symbol. The body walker names nested function declarations and hook-bound arrows (const x = useCallback(…)), but a plain declarator-bound arrow or function expression fell through, so the handler was absent from callers / callees / impact ("Symbol not found", which reads exactly like "no callers") and its calls attributed to the component. The same declaration at module scope already names a function.

Change

• Body walker (wasm) and kernel: an anonymous arrow_function / function_expression that is the whole value of a variable_declarator with a plain identifier name is extracted as a function, contained by the enclosing one, with its own calls. extractFunction already resolves the name from the declarator. A destructuring binding, an inline JSX arrow and a non-function value stay as they were. JS family only.
• torture.tsx gains the shape so kernel-tsjs-parity pins both arms.

Graph shape

A navigation such a handler makes is now the handler's own navigates edge, and the handler is a hop in the Screens via chain — the shape a useCallback handler already has (see the Next.js test's navs(sym('handleSubmit')) and push.via). The react-router and expo-router expectations are updated to that convention; the screen still reaches the handler through its onSubmit={…} reference and the links resolve as before, with the trigger now visible in Steps.

Verification

• New __tests__/nested-declarator-functions.test.ts: arrow, function expression and let-bound arrow become Widget::name functions with their own calls and containment; a value, a destructuring and an inline arrow do not; Python is untouched.
• npm run build:kernel && npm run build, full suite with CODEGRAPH_KERNEL_EXPECT=1: 237 files, 4233 passed, 9 skipped.

Re-index after upgrading to pick up the new symbols.

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