Add StringBuilderExtensions, consolidate StringBuilder helpers

[ivandrofly]

Summary

• New Nikse.SubtitleEdit.Core.Common.StringBuilderExtensions class consolidating generic StringBuilder helpers that were scattered as private/protected statics
• Trim() — moved from SubtitleFormat.TrimBuilder (in-place trim, avoids the sb.ToString().Trim() extra allocation); call sites in SubRip and AdvancedSubStationAlpha updated
• StartsWith(char) / EndsWith(char) — moved (not copied, to avoid ambiguous-call errors) from StringExtensions; existing call sites unaffected since the namespace is unchanged
• CountChar(char) — moved from MergeAndSplitHelper; StringBuilder.GetChunks() turned out to be missing from the netstandard2.1 reference assemblies, so that target falls back to an indexer loop behind the existing #if NET8_0_OR_GREATER guard pattern
• AppendNumber(int, int) — moved from AdvancedSubStationAlpha (zero-padded append matching "{0:00}")
• Adds StringBuilderExtensionsTest (21 tests), including a cross-chunk CountChar case
• Behavior is unchanged; bodies were moved verbatim. Format-specific helpers (AppendTimeCode, EBU/MP4 tag helpers) stay where they are

Test plan

• dotnet build src/libse/LibSE.csproj — both netstandard2.1 and net10.0 targets compile
• dotnet test tests/libse/LibSETests.csproj — full suite passes (956/956 locally)
• Save a subtitle as SubRip and as ASSA and confirm output is unchanged (leading/trailing whitespace trimmed, ASSA time codes still h:mm:ss.cc)

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR consolidates scattered StringBuilder helper logic into a new Nikse.SubtitleEdit.Core.Common.StringBuilderExtensions extension class, and updates call sites to use these shared helpers while adding unit tests to validate behavior.

Changes:

• Added StringBuilderExtensions with Trim, StartsWith/EndsWith(char), CountChar(char), and AppendNumber(int,int) helpers.
• Updated format writers (SubRip, AdvancedSubStationAlpha) and translation merge logic (MergeAndSplitHelper) to use the new extensions.
• Added StringBuilderExtensionsTest coverage for the new helpers.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/libse/Core/StringBuilderExtensionsTest.cs	Adds unit tests for the new StringBuilder extension methods.
src/libuilogic/Translate/MergeAndSplitHelper.cs	Switches merged-row end-character counting to StringBuilder.CountChar.
src/libse/SubtitleFormats/SubtitleFormat.cs	Removes the previous protected TrimBuilder helper (now consolidated into extensions).
src/libse/SubtitleFormats/SubRip.cs	Replaces TrimBuilder(sb) with sb.Trim().
src/libse/SubtitleFormats/AdvancedSubStationAlpha.cs	Replaces TrimBuilder/AppendNumber helpers with sb.Trim() and sb.AppendNumber(...).
src/libse/Common/StringExtensions.cs	Removes StringBuilder-specific StartsWith/EndsWith extensions (moved to StringBuilderExtensions).
src/libse/Common/StringBuilderExtensions.cs	Introduces the consolidated StringBuilder extension methods.

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