Fix integer division and boundary errors in scored-SGF near-end detection

[22nsuk]

In the ownership-based near-end check for scored SGFs, all three counters and boardArea are integers. As a result, count / boardArea truncates before comparison with 0.10 + 6.0 / boardArea. For example, on a 19x19 board the ownership gate accepts 100 unsettled points because 100 / 361 is zero, although the intended strict threshold permits at most 42. The preceding score/win-loss checks still apply independently.

This patch evaluates the intended condition exactly for positive board areas:

10LL * count < (int64_t)boardArea + 60

Widening before multiplication and addition avoids integer truncation and floating-point boundary drift. A double-only fix can still accept the exact boundary on some rectangular boards; for a 10x12 board, 18 unsettled points must be rejected.

The three ownership comparisons are factored into a file-local isNearEndByOwnership() helper shared by the production path and the native regression tests. Ownership counting, score/win-loss checks, subsequent cleanup and target handling, and the existing NoValueScoredNotNearEnding path retain their behavior. A failed near-end check does not unconditionally discard the entire SGF.

Validation

• The native test sweeps each counter independently, plus all three counters together, from zero through the board area on ten board shapes: 9x9, 10x10, 13x13, 19x19, and six rectangles. It makes 6,524 assertions using explicitly specified expected boundary counts.
• Boundary regression validation confirmed that the earlier double comparison fails the rectangular boundary cases. The run also records a separate upstream visit-cap output-test failure and is not presented as an overall passing run.
• Standalone candidate build: the exact upstream-only candidate 90126b1a0afa005927f759c2b2334351b30e53f0 passed Eigen Release runtests and runoutputtests.
• Combined validation: this patch together with the terminal-leaf test correction in Fix visit-cap test invariants for terminal leaves #1258 was rebuilt and passed runtests plus five runoutputtests repetitions.

Related test correction

Standalone repeated output testing exposed a pre-existing visit-cap test invariant that can reject terminal leaves. A deterministic fixture reproduces that failure without this arithmetic change or any production engine changes. #1258 addresses it separately and would preferably merge first. This branch does not include that test correction and targets master independently.

The patch changes only cpp/command/writetrainingdata.cpp, cpp/command/runtests.cpp, and cpp/tests/tests.h, based on d91ea855110dae533f0aada947b2b7d78cc8a4e1. Validation covers the production predicate and native engine tests; it does not include GPU inference or an end-to-end SGF-to-NPZ conversion/dataset impact study.