iris-gui: port live MIPS estimate to CyclesPtr - #77

Problem

iris-gui doesn't compile on current main:

error[E0308]: mismatched types
   --> iris-gui/src/handle.rs:409:34
    |
409 |                         cycles = m.get_rex3().map(|r| r.cycles.clone());
    |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `Option<Arc<Atomic<u64>>>`, found `Option<Cell<CyclesPtr>>`
error: could not compile `iris-gui` (bin "iris-gui") due to 1 previous error

926d56f ("convert cycles atomic into regular volatile variable") changed Rex3::cycles from Arc<AtomicU64> to Cell<CyclesPtr> across 11 files under src/, but iris-gui/ wasn't updated. The GUI's worker loop uses that counter for its live MIPS estimate, so it was still calling .clone() on the Arc and .load(Ordering::Relaxed) to read it.

The CLI (iris, iris-ci) builds fine — only the GUI is affected.

Fix

Latch the CyclesPtr by value rather than cloning an Arc, and read it with .get():

let mut cycles: Option<iris::mips_core::CyclesPtr> = None;
...
cycles = m.get_rex3().map(|r| r.cycles.get());
prev_cycles = cycles.map(|c| c.get()).unwrap_or(0);

This mirrors REX3's own refresh thread at src/rex3.rs:3863 (self.cycles.get().get() — Cell::get for the pointer, then CyclesPtr::get for the count).

Notes on correctness:

• CyclesPtr is Copy/Send/Sync and its doc comment names cross-thread status displays as the intended consumer, valid for the process lifetime. The GUI's MIPS readout is exactly that — it exists because the GUI never runs REX3's own refresh/status-bar loop.
• Being Copy, the Option no longer needs .as_ref().
• .get() is null-safe (returns 0 while dangling), and the pointer is already wired up by the time the GUI latches it, since that happens after Machine::new returns.
• The existing Stop/SyncDisks paths already reset cycles = None before the machine is dropped, so no dangling read.

Testing

./build.sh on macOS arm64 (rustc 1.99.0-nightly), CLI features lightning,rex-jit,tlbvmap,idle-pause and GUI features iris/lightning,iris/idle-pause: all three binaries build clean. No new warnings.