feat: throttled live preview during Record + Z-Stack acquisitions

[hongquanli]

Stacked on #564 (merges after it; rebase onto master if #564 lands squashed).

Shows frames in the live display during Record + Z-Stack acquisitions, which were previously display-blind:

• Recording phase: throttled tap in StreamingCapture (display_fn + display_fps), first frame always shown, then at RECORD_ZSTACK_DISPLAY_FPS (default 3 fps, _def.py/INI; 0 disables preview for both phases).
• Z-stack phase: the inherited per-plane signal_new_image callback (previously wired to a no-op) now emits each captured plane.
• Both paths funnel into QtRecordZStackController.image_to_display, connected to the same display slots as multipointController.image_to_display in every display config (no double-fire — verified against updateNapariConnections's napariLiveWidget skip).
• No Qt in acquisition code: streaming_capture.py / record_zstack_worker.py / record_zstack_controller.py gain only plain callables/floats; Signal appears only in gui_hcs.py. Headless path unchanged (defaults None/0).
• Preview can never harm a recording: a failing display callable logs once and disables preview for that capture; emitted count and zarr sealing are untouched (test-pinned).

Testing

• 7 new tests (3 StreamingCapture throttle/error/off, 2 worker integration incl. pinning the per-plane callback at exactly planes×channels calls, 2 GUI funnel/zero-fps) — 154 tests green across the four affected suites (QT_API=pyqt5).
• Per-task + whole-branch code review: no Critical/Important findings.
• Sim-GUI E2E: real acquisition at Current Position, 1.5 s @ 10 fps → 4 preview frames reached the display signal (expected ~4 at 3 fps).

Design: AI-docs/Squid/to-do/2026-07-06-record-zstack-live-preview-design.md

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