Modernize the bool + out-param return pattern across the public API

[shimat]

Summary

Several public API methods use the C-style pattern bool MethodName(..., out T result, ...), where the bool return indicates success/failure and the actual result comes back via an out parameter. This came up during review of the imgcodecs #2009 coverage work (Cv2.ImReadMulti, Cv2.ImDecodeMulti, etc.) and is worth addressing more broadly rather than one-off per method.

Rough scope (~22 methods across 12 files, from a quick survey):

File	Count	Examples
Cv2_imgcodecs.cs	8	ImReadMulti, ImDecodeMulti, ImEncode (x2), ImEncodeAnimation, ImEncodeWithMetadata, ImEncodeMulti
Modules/objdetect/QRCodeDetector.cs	4	Detect, DetectMulti, DecodeMulti (x2)
Cv2_calib.cs	2	FindChessboardCorners, FindCirclesGrid (out overloads)
Cv2_core.cs	1	CheckRange
Cv2_stereo.cs	1	StereoRectifyUncalibrated
Cv2_objdetect.cs	1	FindChessboardCornersSB (out overload)
Modules/videoio/VideoCapture.cs	1	WaitAny
Modules/aruco/Dictionary.cs	1	Identify
Modules/imgproc/FontFace.cs	1	GetInstance
Modules/saliency/ObjectnessBING.cs	1	ComputeSaliency
Modules/face/Facemark/Facemark.cs	1	Fit

No method in the public API is currently named TryXxx (the only TryGetValue-style member is MatOfT<T>.TryGetValue, which already follows the idiomatic try-pattern). So there's no existing convention to extend here — this issue would be establishing one.

Problem

• The shape itself is dated versus idiomatic modern C# (throw-on-failure, return a default/empty value on failure, or an explicitly-named TryXxx method).
• Argument validation is inconsistent across methods that otherwise look similar. For example, in Cv2_imgcodecs.cs, ImReadMulti/ImReadWithMetadata/ImReadAnimation only null-check the filename (ArgumentNullException.ThrowIfNull), while ImRead/ImWrite/ImEncode reject empty strings too (string.IsNullOrEmpty) — so Cv2.ImReadMulti("", out mats) silently returns false where Cv2.ImRead("") throws.

Proposal (needs discussion, not decided yet)

Options to consider per method/family:

1. Return T directly, with an empty/default value signaling failure (matches Mat's existing "return empty Mat on failure" convention used by ImRead/ImDecode).
2. Keep the bool-return shape but rename to TryXxx to make the contract explicit (matches BCL convention, e.g. int.TryParse).
3. Throw on failure instead of returning a sentinel/bool (bigger behavior change, more invasive at call sites).

Given OpenCvSharp5 is a major-version line where breaking API changes are acceptable, any of the above is on the table. This issue is to track the discussion and decide a consistent policy before touching all ~22 methods.

Scope note

The imgcodecs null/empty-check inconsistency mentioned above is being fixed narrowly (just the check, not the out+bool shape itself) as part of the #2009 imgcodecs coverage PR. This issue is about the broader shape decision across the whole public API, which is intentionally kept out of that PR.