Continuation style: fix custom style not reaching engine, store per profile

[niksedk]

Summary

Fixes the custom continuation style not reaching the fix/merge engine, and makes the custom continuation style per rule profile instead of a single global setting.

The core bug

The libse fix/merge engine reads the custom continuation style from Configuration.Settings.General (14 fields + ContinuationPause). The SE5 settings bridge only copied the ContinuationStyle enum there and never the custom fields — so selecting the Custom style silently fell back to libse defaults in Fix Common Errors and Merge. Presets were unaffected (only the enum matters for them).

Per-profile storage + refactor (#2/#3)

• New CustomContinuationStyle value object (src/libse/Common/CustomContinuationStyle.cs) collapses the 14 flat fields into one object, used by RulesProfile, ProfileDisplay, and SeGeneral.
• The flat-field mapping the engine reads is centralized on the value object (ApplyToGeneralSettings / FromGeneralSettings).
• The active profile is mirrored onto SeGeneral and bridged to libse via a single Se.ApplyContinuationStyleToLibSe(), called from both bridge sites (MainViewModel.SetLibSeSettings and FixCommonErrorsViewModel.InitStep1).
• Adding a future field now touches the value object + libse GeneralSettings/engine + the one mapping, instead of ~12 hand-copied sites. Net −119 lines in SettingsViewModel.

Additional fixes

• "Edit custom style" button not appearing when the settings page was reopened with a Custom-style profile.
• Continuation settings not reset by Reset Rules.
• ColorDurationTooLong not reset by Reset Syntax Coloring (self-assignment no-op).

Notes

• No migration is performed for the old global custom style (this is pre-release; an upgrading tester's global custom values reset to defaults). Deliberate, per discussion.
• The legacy RulesProfile.Serialize/Deserialize (used by old SE, not SE5) does not carry the per-profile custom style — SE5 persists it via System.Text.Json.
• Null-safety hardened on the engine path (null-tolerant copy-ctor, bridge coalesce, Se.SetDefaultValues guards mirror + profiles).

Tests

• libse: CustomContinuationStyle (defaults match legacy, deep copy, null-tolerant copy, ApplyTo/From round-trip), RulesProfile (copy/serialize).
• UI: Se.ApplyContinuationStyleToLibSe() bridges into libse Configuration; per-profile custom style survives a real SaveSettings→LoadSettings round-trip.
• Full suites pass: libse 383, UI 333. Solution builds clean.

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