Make NNUE eval the default, same convention as the EGTB path default

main.c's InitializeOptions now tries to load "eval.nnue" (a file next
to the binary, matching typhoon_nnue's own NNUE_WEIGHTS default --
typically a symlink into head_reference/nnue/current/weights.nnue)
after argument parsing, if the user hasn't already enabled NNUE
explicitly via --nnueweights or an interactive "nnue load"/"nnue on".
Same hardcoded-default-path convention main.c already uses for
--egtbpath (/zscratch/egtb).

Fails soft: a checkout without eval.nnue present, or one where it
fails to load, just falls back to HCE eval exactly as before -- this
is a default, not a requirement, so there's no UtilPanic the way
--nnueweights's own explicit-request failure path has. The existing
interactive "nnue off" command remains the way to explicitly opt back
out of NNUE for a given session; a script that wants guaranteed HCE
(e.g. an apples-to-apples eval comparison) should add "nnue off;" to
its own --command string rather than relying on the old no-flag
default.

Verified: clean release build, clean DEBUG=1 TEST=1 self-test run (no
assertion failures), and manually confirmed both the default-load path
(eval.nnue present in cwd) and the silent-fallback path (run from a
directory without it) behave as expected; --nnueweights still works
unchanged.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

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