CommonJS model: Rule 2 misses unions with absent, loses reasons in union(), and treats depth cutoffs as complete #32

Found in the PR #29 round-14 review. Rule 2 bails a file when an unknown is reachable from its exports, but some incomplete answers are not unknowns:

1. Union with absent. function run(){} if (typeof window === 'undefined') exports.run = run; gives run = union[run, absent]. That member is not unknown, so the file stays modelled, the union select rejects it, and the pre-PR edge go → m.js:run is lost.
2. union() drops the reason. CommonJsValueGraph.union returns unknown('unknown alternative'). A conditional if (process.env.X) patch(exports) therefore bails to the pre-PR graph, with its wrong decoy edges, while the unconditional patch(exports) stays ruled no-edge. Whether the opaque-call ruling applies depends on the if.
3. Depth cutoffs count as complete. forwardsOutside (seen.size >= 8) and incomplete() (depth > 8) treat their cutoff as "inside the model". A chain f0→…→f10 of module.exports = require(...) ending in ESM loses go → f10.js:run.

Fix direction: count absent-in-union as incomplete; keep a blocking reason through union (e.g. a set of reasons); treat any cutoff as unmodelled.