Add -flto and -mtune=native to the release build

-flto: several functions (_RookAttacksBB/_BishopAttacksBB in
generate.c, and others from recent sessions) are declared FORCEINLINE
but non-static, exposed specifically so other translation units can
call them -- __attribute__((always_inline)) only actually forces
inlining within the TU that has the function body, so every cross-file
call was paying for a real call/ret instead of inlining without this.

Measured via the `bench` command (4 reps each, interleaved with a
non-LTO baseline): a clean, reproducible ~6.6% NPS improvement with
zero overlap between the two distributions (baseline 1643k-1661k nps,
LTO 1752k-1768k nps) -- not noise.

-mtune=native: tunes instruction scheduling/costs for the actual build
machine instead of a generic x86-64 target. Measured weaker and
noisier (~1.5%, some overlap with baseline) but real and free -- no
portability cost the way -march=native's newer-ISA-extension
dependency would carry.

precommit_check.sh (self-tests + DEBUG smoke test) passed clean with
both flags active.

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

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