Visitor and evaluator edge cases can over-prune files or mishandle nulls

Several visitor/evaluator edge cases appear unsafe or inconsistent:

1. `_StrictMetricsEvaluator.visit_not_equal` / `visit_not_in` return `ROWS_MUST_MATCH` when a file can contain nulls or NaNs. Example stats with `[null, 5]` or `[NaN, 5.0]` and lower/upper bounds both `5` return true for `NotEqualTo("x", 5)` / `NotIn("x", {5})`, even though one row does not match. This can incorrectly mark whole files deleted.

2. `_StrictMetricsEvaluator.eval` returns `ROWS_MUST_MATCH` for `record_count <= 0`. `record_count=0` is vacuously true, but `record_count=-1` is unknown per the local comment; even `AlwaysFalse()` returns true.

3. `ResidualVisitor` comparison methods directly compare partition values to literals. A nullable identity partition value of `None` with `LessThan("x", 1)` raises `TypeError`, while row evaluation returns false.

4. `ResidualVisitor.visit_not_nan(None)` returns `AlwaysFalse`, while expression evaluation treats `NotNaN(None)` as true. Existing tests encode both behaviors, so the semantics are inconsistent.

Validated against the current tree; examples use stats/partition shapes already supported by the repo tests.