CommonJS model: destructuring and for-in/of targets that write exports are not seen #31

Found in the PR #29 round-14 review. Rule 1's write detection only recognises = / compound assignment and ++/--, and write() ignores a non-member LHS. Destructuring and for-in/for-of targets that write an exports member are therefore read as no write: the file stays modelled and the member answers absent.

Repros (each loses the pre-PR edge run → m.js:b):

• exports.a=a; [exports.b]=[b];
• ({x: exports.b} = {x: b});
• for (exports.b of [b]); and for (exports[k] in o);
• nested: function init(){ [exports.b]=[b]; } init();

Fix direction: treat any binding/assignment-pattern or for-in/of initializer target rooted at exports (or an exported value) as a write; at top level model it or bail, and when nested, bail.