common-controller-scripts: add script.getPlaypositionSamples

[gary-gdbsystems]

Summary

Resolves #12950.

Adds a mapping script helper script.getPlaypositionSamples(group) that returns the current play position of a group as an absolute engine sample position — the same unit as loop_end_position and other absolute position controls — computed from the relative playposition control and track_samples:

script.getPlaypositionSamples = function(group) {
    return engine.getValue(group, "playposition") * engine.getValue(group, "track_samples");
};

Why a script helper (not a ControlObject or C++ API)

In the issue thread, @Holzhaus and @ronso0 explicitly chose a JavaScript helper over adding a new sample-based ControlObject:

@Holzhaus: "I'd rather not add more sample-based COs when we plan to switch to frame-based COs... If this is just about scripts, we could also add a JS helper method instead."
@ronso0: "Agreed, let's go with the script helper."

This PR implements exactly that, and supersedes the closed #16786, which added a C++ engine.getPlaypositionSamples() method instead of the requested script helper.

Changes

• res/controllers/common-controller-scripts.js — one helper function (+8 lines).

Testing

• ControllerScriptEngineLegacyTest passes (41/41), including commonScriptHasNoErrors, which evaluates the full common-controller-scripts.js in the JS engine.
• eslint and the other applicable pre-commit hooks pass.