docs: refresh MathEditorSwift regression inventory

[maitbayev]

Motivation

• Refresh the regression inventory to reflect that the previously reported guard/early-return insertion regressions in the Swift editor have been addressed and to narrow the active parity gaps to the remaining issues.

Description

• Update MathEditorSwift/regressions.md to mark insertion-related regressions as resolved and to retain the outstanding parity gaps, based on a direct review of MathEditorSwift/Sources/MathEditorSwift/MTEditableMathLabelSwift.swift (noting use of resolvedInsertionIndex() and direct label invalidation) and the legacy mathEditor/editor/MTEditableMathLabel.m behavior.

Testing

• Ran swift test in MathEditorSwift, which failed to run due to SwiftPM being unable to fetch the https://github.com/maitbayev/iosMath.git dependency (network/403), so no automated tests completed successfully.

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Codex Task