fix(extraction): index const-bound functions inside a body as symbols (#1669)

[colbymchenry]

Fixes #1669.

Lands / rebases #1679 onto current main (CHANGELOG conflict with #1772 / #1674+#1639 only).

Problem

const handleClear = () => {…} inside a component — every React handler written without useCallback — was never a symbol. The body walker named 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.
• React-router / expo-router expectations updated so a navigation such a handler makes is the handler's own navigates edge (same shape a useCallback handler already has).

#1718 / fuzzy lexical reachability is already on main — nested handlers become real symbols without becoming fuzzy false positives across files.

Verification (Linux)

• Fail→pass: pre-fix dist: handleClear missing, handleSelect (useCallback) present. Post-fix: both indexed as Widget::handleClear / Widget::handleSelect.
• E2E issue repro: codegraph init → 12 nodes; callers handleClear → Widget; query kind:function lists both handlers.
• CODEGRAPH_KERNEL_EXPECT=1 vitest: nested-declarator-functions, react-router, expo-router, kernel-tsjs-parity, fuzzy-lexical-reach — all passed.

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