CommonJS model: loops, switch, short-circuit and try writes are modelled as straight-line #30

Found in the PR #29 round-14 review. The CommonJS value model walks top-level code as straight-line, so a write that may never run replaces the real export, and the file stays modelled with a wrong answer. Only if and logical assignment fork the graph.

Repro: m.js function run(){} exports.run = run; for (const k of []) exports.run = 42; with c.js const {run} = require('./m'); const lib = require('./m'); function go(){ run(); lib.run(); }.

• Pre-PR: go → m.js:run (x2).
• PR29: no edge, because run is modelled as data 42.

Same with switch arms, cond && (exports.run = 1), ?: and try bodies written after the real write.

Fix direction: fork the graph for these constructs, or treat an exports write inside them as an unmodelled use (Rule 1), which makes the file resolve as pre-PR.