Fix all build warnings across release/DEBUG/TEST profiles

Clean gmake GENETIC=1 PERF_COUNTERS=1 MP=1 SIXTYFOUR=1 build had 100
warnings; DEBUG=1 and TEST=1 builds had more once actually exercised.

- OFFSET_OF/CONTAINING_STRUCT (chess.h) and PTR_TO_ALLOC_HASH (unix.c)
  truncated pointers through 32-bit ULONG before use in offset/hash
  arithmetic on this 64-bit build -- routed through size_t instead.
- Diagnostic int<->void* round-trips (command.c, root.c, split.c,
  sig.c, data.c, unix.c, util.c) widened/narrowed via size_t to avoid
  implicit truncation.
- ABS_DIFF on unsigned COOR now casts to int before abs().
- Dropped -fexpensive-optimizations (GCC-only, clang silently ignores
  it) from GNUmakefile.
- Removed genuinely dead variables (book.c, gamelist.c, split.c,
  testgenerate.c, testhash.c).
- Guarded DEBUG/PERF_COUNTERS/_X86_-only variables and the
  _CMEvidenceBucket helper under the #ifdef that actually reads them,
  since ASSERT/EVAL_TERM/KEEP_TRACK_OF_FIRST_MOVE_FHs compile away
  outside those builds.
- Added missing prototypes for SlidePawn, SlidePawnWithoutSigs,
  SlidePieceWithoutSigs (move.c), previously undeclared in chess.h.
- Removed dead _SystemIsRoot (unix.c).

Verified via precommit_check.sh: TEST=1 self-test suite passes,
DEBUG=1 smoke test (10 random ECM positions, sd 4) passes with no
crashes/assertions, release build restored -- all three profiles now
build with zero warnings.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_014Cmv11sJZqVfanrPh6UnWE

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