Persist user's spell/grammar toggle across caret movement

[Nicolas-Py]

Closes #34.

Summary

• updateAutocorrectSettings no longer overwrites the user's manual "off" when the caret leaves a code/LaTeX/link suppress zone. The restore branch reads a per-coordinator preference instead of hardcoded true.
• New SpellCheckingPolicy on MarkdownEditorConfiguration lets embedders seed initial state. New onSpellCheckingPolicyChanged callback on NativeTextViewWrapper lets them persist user toggles (e.g. to UserDefaults) and feed the value back on next launch.
• NativeTextView overrides toggleContinuousSpellChecking(_:), toggleGrammarChecking(_:), and toggleAutomaticSpellingCorrection(_:) to capture context-menu toggles into the coordinator state.

Defaults match prior behavior — SpellCheckingPolicy.default is all-on, so existing callers see no change.

Files

• Configuration/MarkdownEditorConfiguration.swift — SpellCheckingPolicy struct + spellChecking field.
• TextView/Coordinator/NativeTextViewCoordinator.swift — userPrefers* mirror, onSpellCheckingPolicyChanged callback, didToggleSpellCheckingPolicy(textView:) handler.
• TextView/Coordinator/NativeTextViewCoordinator+Autocorrect.swift — restore branch honors userPrefers*. Suppress-zone path unchanged.
• TextView/NativeTextView/NativeTextView+SpellingToggles.swift — new file, AppKit toggle action overrides.
• TextView/NativeTextViewWrapper.swift — makeNSView reads from configuration.spellChecking; new init param threads the callback through makeCoordinator.

Embedder usage

var config = MarkdownEditorConfiguration.default
config.spellChecking = SpellCheckingPolicy(
    continuousSpellChecking: UserDefaults.standard.object(forKey: "spell") as? Bool ?? true,
    grammarChecking: UserDefaults.standard.object(forKey: "grammar") as? Bool ?? true,
    automaticSpellingCorrection: UserDefaults.standard.object(forKey: "autocorrect") as? Bool ?? true
)

NativeTextViewWrapper(
    text: $text,
    configuration: config,
    onSpellCheckingPolicyChanged: { policy in
        UserDefaults.standard.set(policy.continuousSpellChecking, forKey: "spell")
        UserDefaults.standard.set(policy.grammarChecking, forKey: "grammar")
        UserDefaults.standard.set(policy.automaticSpellingCorrection, forKey: "autocorrect")
    }
)

Test plan

• Open a doc with a code block. Disable "Check Spelling While Typing" via context menu. Type across the code-block boundary and confirm the underline does not return.
• Same flow for "Check Grammar With Spelling" and "Correct Spelling Automatically".
• With an embedder wired to UserDefaults: relaunch the app and confirm the previous off-state survives.
• Default config (no spellChecking override) behaves identically to main — all three toggles on at launch.
• swift build and swift test clean.

Out of scope

Smart Quotes (isAutomaticQuoteSubstitutionEnabled) has the same caret-movement bug pattern. Tracked separately.

[luca-chen198]

gj