[patch] Keep string, float and long literals through a YAML round trip

[matt-edmondson]

Fixes #83

What changed

String literals. YamlSerializer let the emitter choose whether to quote a string. It already quoted true and 123, but it wrote null, Null and ~ bare, so they came back as ''. The serializer is now built with WithQuotingNecessaryStrings(), which quotes any string the reader would take for a non-string scalar.

Float and long literals. The issue offered two options: support these end to end, or remove Literal.SingleValue and Literal.BigNumberValue. Removing them would break the public API, so this PR supports them.

• YamlSerializer has a case for LiteralExpression<float> and LiteralExpression<long>. Before, both were written as {}.
• YamlDeserializer reads Literal<Single> and Literal<Int64>. A float is parsed as a float directly, which avoids rounding twice. It also accepts the .inf, -.inf and .nan spellings: the non-finite parsing moved out of TryParseDouble into a shared TryParseNonFinite.
• In LanguageGeneratorBase:
  • TryGenerateCommonNode writes both types. A new FormatSingle formats a float as a float, so 0.1f stays 0.1 instead of widening to 0.10000000149011612. It adds .0 to a whole number the same way FormatDouble does.
  • CanGenerateStandardNodes and LeadingSign know both types, so -(-5000000000) is not written as --.
• CSharpGenerator adds the f and L suffixes.
• GoGenerator treats both as compile-time values and infers float32 and int64 for them.

Not changed: the graph editor (Coder.Graph/AstFields, AstSchema) still has no editable field for float or long literals. That is a separate UI change. This PR is about data loss on save and load.

Tests

New file: Coder.Test/Serialization/LiteralRoundTripTests.cs.

• String round trip for null, Null, ~, true, False, 123, -4.5, 1e3, .inf, .nan, the empty string and plain text.
• Float round trip for finite, whole-number, MaxValue and non-finite values.
• Long round trip, including MaxValue and MinValue, and as a variable initialiser.
• All seven generators write float and long literals. C# writes the suffixes, a whole-number float keeps .0, and a negated negative long doesn't fuse its signs.

With the Coder/ changes reverted, 19 of the 28 new cases fail. The 9 that pass are strings the emitter already quoted. With the fix, the full suite passes (962/962) and dotnet build Coder.sln reports no errors.

🤖 Generated with Claude Code

https://claude.ai/code/session_01YLYtHwwqdY3Tk9vpkwT6Sw

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Generated by Claude Code

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Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
89.3% Coverage on New Code
0.0% Duplication on New Code

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