feat(eventviewer): add EventViewer.exportImage() PNG/JPEG export (#321)

[HanSur94]

What was built

Adds a new additive public method exportImage(obj, filepath, format) to EventViewer (libs/EventDetection/EventViewer.m), so the interactive event-analysis figure can be saved straight to a report image — closing the last figure-producing surface in FastSense without a save path.

v = EventViewer(events);
v.exportImage('events.png');          % format inferred from extension
v.exportImage('events.png', 'png');
v.exportImage('events.jpg', 'jpeg');  % 'jpg' alias accepted

Captures obj.hFigure at 150 DPI; PNG default, JPEG by extension/argument.

Design — port, don't reinvent: the method ports DashboardEngine.exportImage's hardened cross-platform branch verbatim (exportapp on MATLAB R2024a+ / exportgraphics / getframe+imwrite fallback; print()+hidden-stub-axes on Octave), retargeting obj.hFigure and swapping the three error IDs to the EventViewer: namespace. No DashboardEngine: IDs remain in the file.

Scope kept minimal: the method is the deliverable; the optional toolbar "Export Image" button is explicitly a follow-on in the issue and is not included — so the change touches one method plus its tests, with no toolbar-layout edits.

Backward-compatible & additive only: one new public method; no signature, Event/EventStore/DashboardWidget/Tag contract, or serialization change (EventViewer is not serialized). Toolbox-free, pure MATLAB/Octave, no MEX.

Error IDs: EventViewer:notRendered (figure empty/closed), EventViewer:unknownImageFormat, EventViewer:imageWriteFailed.

How it was verified

• TestEventViewerExtras: 16/16 pass in the live MATLAB session (4 new cases: PNG round-trip exists+non-empty, JPEG via explicit format + extension inference, unknownImageFormat error, notRendered-after-close error — all reusing the existing makeFixtureEvents/safeClose/safeDelete fixtures with tempname+addTeardown).
• TestEventViewer: 7/7 pass — no regression in the base suite.
• Real export round-trip (live): valid 1200×700 PNG (27.5 KB, confirmed via imfinfo) + JPEG via extension inference; both error IDs fire correctly.
• MISS_HIT mh_style + mh_lint: everything seems fine on both changed files.

Closes #321

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AI-built via /feature-build — needs human review before merge.

[codecov]

Codecov Report

❌ Patch coverage is 21.31148% with 48 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
libs/EventDetection/EventViewer.m	21.31%	48 Missing ⚠️

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[HanSur94]

Wiki documentation updated for EventViewer.exportImage():

• The auto-generated API-Reference: Event Detection page already picked up the new method via the doc-gen bot (commit c5d8c02).
• The hand-written Event Detection Guide EventViewer section is updated in docs(wiki): document EventViewer.exportImage in Event Detection guide #373 to show exportImage() usage and correct a stale "context menu export" bullet.

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