Fix #1729: Status bar cursor updates without requiring a tab switch

[Borisserz]

Description

Fixes the status bar Line/Col label staying empty (or stale) until the first editor tab switch after opening a workspace.

Root causes addressed:

• Editor.openTab could select a freshly allocated EditorInstance while tabs kept a different object for the same file (EditorInstance equality is by file). The editor view and status bar then observed different publishers.
• CodeFileView wrote newState.cursorPositions ?? [], clearing the caret on scroll-only SourceEditor updates that omit cursor state.
• The status bar label now tracks the active instance by object identity, seeds from the current caret, and falls back safely when line/column are unresolved.

Also documents the behavior in DocC and adds unit + UI regression coverage.

Related Issues

• Closes 🐞 Status Bar does not update when workspace is first opened #1729

Checklist

• I read and understood the contributing guide as well as the code of conduct
• The issues this PR addresses are related to each other
• My changes generate no new warnings
• My code builds and runs on my machine
• My changes are all related to the related issue above
• I documented my code

Screenshots

N/A for this logic fix (status bar text). UI regression covered in ProjectNavigatorUITests.

Test plan

• StatusBarCursorPositionLabelTests (label formatting / unresolved fallback)
• EditorTabReuseTests (reopen reuses same EditorInstance)
• SwiftLint --strict on touched files (0 violations)
• Manual: open workspace from Welcome → Line/Col visible without switching tabs; clicking in editor updates the label
• CI SwiftLint + tests green

Made with Cursor