CommonJS model: writes to non-writable or frozen exports are modelled as taking effect #40

Found in the round-1 review of branch fix/commonjs-model-gaps (stacked on PR #29). It is probably older than that branch, since the model code involved is unchanged there.

In sloppy mode, a write to a non-writable or frozen property is silently ignored, but the model applies it:

• Object.defineProperty(exports, 'run', { value: a }); exports.run = b; gives go -> b, while runtime is a. The same happens with { value: a, writable: false }.
• exports.run = a; Object.freeze(exports); exports.run = b; gives go -> b, while runtime is a.
• Baseline has no edge for either.

Fix direction: track writable: false and frozen state on the owned object, and drop later writes (or send the file to the pre-model path).