Retag `||` on the truthy side instead of coercing the left

- tsc builds `u || d`'s result by DROPPING the left's falsy arms (`process.env.X || null` is `string | null`, `|| 3000` is `string | number`), so coercing the left into that result before the truthiness test retags an arm the test is about to rule out — and threw where Node yields the default.
- `orDefault` gains an optional `retag`: the truthy side hands the whole left box to a union→union helper whose stranded-arm throw is unreachable by construction, since truthiness has already ruled those arms out. Left evaluates once, right stays lazy, arm count is free.
- A ternary in logical-left position no longer adopts its contextual union over its own, for the same reason the eager coercion was wrong: tsc's assignability proof does not hold for that operand. Narrow to the union choice — contextual array element typing and the unmappable-own rescue are untouched.
- 2595 chained its two in-flight `fs.exists` stats: registering both after the awaited rejections orders them against the rejections, not against each other, and Node itself returns them reversed about one run in forty.