feat(memory): --mem-watch=ADDR per-store writer trace

Adds an opt-in diagnostic that emits one tracing line per guest store
overlapping any armed byte address, naming the writer (tid, pc, lr)
plus old/new u32 lanes. Mirrors the --pc-probe / --branch-probe shape;
pc/lr are stamped from worker_prologue via a thread-local Cell, so
default runs (empty watch set) take a single is_empty() check on each
write. Lockstep digest preserved (instructions=100000003 across reruns,
sylpheed_n50m.json golden byte-identical).

Diagnostic infra only; no functional change. Used to identify producers
of dispatch-state writes for the audit-017 / audit-019 hunt.

crates/xenia-memory/src/lib.rs

@@ -8,7 +8,7 @@ mod platform;
 use thiserror::Error;
 
 pub use access::MemoryAccess;
-pub use heap::{GuestMemory, HeapType};
+pub use heap::{set_writer_ctx, GuestMemory, HeapType};
 pub use mmio::MmioRegion;
 pub use page_table::PageEntry;