perf(memory): unnecessary String allocation in write-gate relation check

[bug-ops]

Description

is_low_signal_relation() in crates/zeph-memory/src/semantic/graph.rs calls .to_lowercase() on relation strings, allocating ~30 short Strings per extraction pass. All entries in the predicate list are ASCII.

Location

crates/zeph-memory/src/semantic/graph.rs — is_low_signal_relation function

Fix

Replace .to_lowercase() == "..." comparisons with eq_ignore_ascii_case("..."). Zero allocation, same semantics for ASCII inputs.

Environment

• Identified during PR feat(memory): Benna-Fusi multi-timescale SYNAPSE edges and MemORAI adaptive retrieval #4697 performance review
• Impact: low; 30 short-string allocations per extraction pass