Fix dead-code warnings and document parameter-search leads

[PaulGregory1]

Two small, low-risk contributions to the editable src/point_add path. No
harness/contract files touched.

1. Fix two dead-code warnings (arith/multiply.rs)

Both flagged by cargo build in square_row_windowed_apply:

• The borrow_lane closure declares b: &mut B but never uses it (it reads
  the captured tmp_ext directly) → renamed to _b.
• let seg_w = hi - lo; at the top of the per-window loop is never read; the
  same value is re-bound and used later in the same loop body → removed.

Behaviour-neutral by construction. Confirmed with the official harness:
./benchmark.sh → score 1,519,170,048 (unchanged), toffoli 1,318,724,
qubits 1,152, and 9,024/9,024 shots OK (phase- and ancilla-clean).

2. Document parameter-search leads (memory/param-search-2026-07.md)

Following the README's suggestion to record approaches under
src/point_add/memory/, this logs an 18-config single/double-knob sweep of
the committed circuit that found no cheap score win — so the next
contributor can skip it. Summary: every qubit-reserve/target reduction fails
validation (scratch is at its floor), and the skip/margin knobs are
non-binding at their current values. A trace also pins down the qubit peak:
the GCD loop tops out at 1,140 active qubits while the global peak is 1,152, so
the ~12 peak-binding qubits live in a non-GCD phase whose reserve is already at
its floor — i.e. the score's x qubits factor needs a structural change there,
not a reserve knob.

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