feat(parser): capture JS/TS member-assigned function expressions

[avzamelek]

Problem

Prototype- and module-augmentation assignments — app.handle = function () {},
Router.prototype.dispatch = () => {}, exports.render = function () {} — are how
Express, Koa and many older JS/TS modules define their entire public API. The parser
only captured const x = fn (variable_declarator) and class fields, so these members
produced no Function node at all — invisible to callers_of, impact, dead-code and
search. This is part of the README's "JavaScript … flow detection needs work", and
matches the note in eval/configs/express.yaml ("JS modules use prototypes +
module.exports heavily, so most methods are not represented").

Change

Add _extract_js_member_functions, mirroring the existing _extract_js_var_functions:
an assignment_expression whose LHS is a member_expression and whose RHS is an
arrow/function value becomes a Function node qualified by its full member path
(file::app.handle, the same shape as file::Class.method), with a CONTAINS edge and
body-call recursion. Computed access (obj[key] = fn) is skipped.

Impact

On expressjs/express this adds 89 Function nodes (1771 → 1860), surfacing the
prototype-augmented public API (app.use, app.handle, app.route, app.param,
req.accepts*, res.*). Scope is structural coverage only; it does not by itself
move flow_completeness recall, which is gated on JS member-call resolution. app.get/
app.post remain uncaptured because express defines them via a computed
methods.forEach(m => app[m] = …) loop, which this handler intentionally skips.

Tests

TestJsMemberAssignedFunctions in tests/test_parser.py (6 cases; fail before, pass
after). Full suite green (1965 passed); ruff check code_review_graph/ clean.

[avzamelek]

Follow-up added: static same-file JS/TS member calls now resolve to member-assigned function definitions.
For example, after app.handle = function () {}, a later app.handle() now creates a CALLS edge to file::app.handle instead of the bare handle target.
The resolver keeps the existing fallback for unresolved external calls such as thirdParty.handle().
Added regression coverage; tests/test_parser.py passes (112 passed).
Dynamic/computed assignments (app[m] = fn) and alias/destructuring resolution remain intentionally out of scope for this PR.

[avzamelek]

Follow-up added for optional chaining.
app?.handle() is now normalized to the same static member path as app.handle(), so when app.handle = fn exists in the same file, the CALLS edge resolves to file::app.handle.
Computed optional access such as app?.key remains intentionally unresolved.
Added regression coverage; tests/test_parser.py passes (113 passed).