Fix batch-mode exit code, killer-table backfill collision, impossible QSearch mate-magnitude asserts, MATEMOVE PV display, --command truncation, and test.sh's stale egtbpath.

Several small, independent correctness fixes bundled together since
they were all exercised together through today's precommit_check.sh
and curated-suite runs:

- command.c: batch-mode's "Exhausted input" exit was exit(-1), which
  truncates to 255 (an 8-bit status) and is indistinguishable from a
  real crash's nonzero exit. Changed to exit(0) so debug_smoke_test.sh
  can reliably tell a clean batch run apart from a crash by exit status
  alone.

- dynamic.c: _NewKillerMove's slot[1] backfill from
  mvNullmoveQuietRefutations[uPly] had no check that the backfilled
  move differed from the move just placed in slot[0]. When they
  coincided, both slots held the identical move, silently wasting a
  killer slot in release builds (ASSERT is a no-op there) and tripping
  _NewKillerMove's own IS_SAME_MOVE invariant in DEBUG builds. Fixed by
  skipping the backfill on collision.

- search.c: removed two ASSERT(iBestScore > -NMATE) calls in QSearch
  that encoded an invariant that isn't actually guaranteed -- at an
  early full-width root iteration, or after aspiration-window widening
  following repeated fail-highs, an ancestor frame's iAlpha/iBeta can
  itself already be more extreme than -NMATE with no mate anywhere in
  the line, so a legitimate fail-low placeholder or fail-high score can
  land in mate-magnitude territory purely as a window artifact.
  hash.c's storage path already treats any value <= -NMATE as a sound
  upper bound regardless of origin, so this was a false invariant, not
  a caught bug. Also: minor whitespace cleanup, an added ASSERT
  documenting the futility-margin depth precondition it replaced a
  redundant runtime check for, and PV/leaf-count bookkeeping on the
  mate/draw-at-root leaf paths that was previously skipped.

- util.c: MATEMOVE sentinel moves weren't handled in PV-to-string
  conversion, so a PV ending in a detected mate would either display
  garbage or hit the same-move assert. Added an explicit "<#>" marker.

- test.sh: --egtbpath pointed at a nonexistent /egtb/three;/egtb/four;
  /egtb/five; corrected to /zscratch/egtb, this box's actual EGTB
  location.

- main.c/input.c: --command's initial-command buffer (g_szInitialCommand)
  was a fixed 256-byte array; strncpy(..., SMALL_STRING_LEN_CHAR - 2)
  silently truncated any longer --command string, and -- worse -- when
  the source was long enough not to fit, strncpy doesn't null-terminate
  the destination, so the immediately-following strcat(..., "\r\n") could
  read/write past the buffer. Long move-replay command strings used
  during this session's debugging hit the truncation directly (a ~600
  char move list silently cut off mid-token, desyncing the input queue).
  Changed g_szInitialCommand to a heap allocation sized to the actual
  input length instead of a fixed cap.

- CLAUDE.md: documents the above (this file's own diff is prior
  session's writeup of these same fixes, committed now alongside the
  code).

All exercised together via precommit_check.sh (self-test suite + DEBUG
smoke test against random ecm.ep_ samples) and the sd10/sn5m curated
suite sweep run for the eval.c hand-tuning commit just before this one.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01M9ZDiJhiUajUxh95mTXCFJ

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