Add opening seed control for selfplay opening randomness

[john-sp]

Adds a new selfplay flag, --opening-seed, to control randomness used by shuffled and random opening modes.

Changes:

• Adds --opening-seed with default -1 to preserve existing nondeterministic behavior.
• Uses the seeded opening RNG for: --openings-mode=shuffled and --openings-mode=random
• Adds a non-breaking Random(int seed) constructor while leaving Random::Get() unchanged.
• Keeps unrelated tournament randomness on the existing global RNG.

[Copilot]

Pull request overview

This PR adds deterministic control over selfplay opening selection by introducing a dedicated RNG seed (--opening-seed) used only for opening-book shuffling and random opening selection, while leaving the existing global RNG behavior unchanged for other tournament randomness.

Changes:

• Add --opening-seed (default -1) to control randomness for --openings-mode=shuffled and --openings-mode=random.
• Introduce a new Random(int seed) constructor that preserves current non-deterministic seeding for negative seeds.
• Use a per-tournament opening_random_ RNG for opening shuffle/selection instead of Random::Get().

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/utils/random.h	Exposes a seeded Random(int seed) constructor to support deterministic RNG instances.
src/utils/random.cc	Implements seeded initialization (negative seed preserves current non-deterministic behavior).
src/selfplay/tournament.h	Adds a dedicated opening_random_ member RNG for opening-related randomness.
src/selfplay/tournament.cc	Adds --opening-seed, initializes opening_random_, and switches shuffled/random opening logic to use it.

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