Fix vertical caret movement

[cyanzhong]

No description provided.

[Copilot]

🟡 Changes recommended

The new vertical-motion logic uses range.assoc || 1, which incorrectly overrides a valid assoc = 0 and can produce incorrect coordinate/selection behavior.

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Pull request overview

This PR introduces a custom “ArrowUp / Shift-ArrowUp” vertical movement command for the hidden-syntax editor mode to correct cases where CodeMirror vertical motion skips over an ATX heading line, and adds targeted tests to lock in the expected behavior.

Changes:

• Added cursorLineUp / selectLineUp commands plus a correctedLineUp keymap extension to adjust upward motion when an ATX heading is skipped.
• Registered the new keymap in the hidden-syntax base extension so it applies when hiddenSyntaxExtension is enabled.
• Added a new “Vertical motion” test suite covering correction behavior, selection-extension semantics, multi-selection fallback behavior, and precedence with higher-priority keymaps.

File summaries

File	Description
tests/hiddenSyntax.test.ts	Adds a “Vertical motion” test suite validating corrected ArrowUp/Shift-ArrowUp behavior and precedence rules.
src/hiddenSyntax/selection.ts	Implements corrected upward vertical motion commands and exports a Prec.high keymap extension.
src/hiddenSyntax/index.ts	Wires the new corrected ArrowUp keymap into the hidden-syntax base extension.

Review details

• Files reviewed: 3/5 changed files
• Comments generated: 1
• Review effort level: Lite

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