Rewriting a document changes unquoted numbers: version: 1.10 becomes 1.1, zip: 01234 becomes 1234, lat: 51.507351 becomes 51.50735 #178

What's wrong

Since #174 (the fix for #137, commit 4504544), YamlSerializer's deserializer uses WithAttemptingUnquotedStringTypeDeserialization() (Frontmatter/YamlSerializer.cs:30). Plain scalars now deserialize to the smallest .NET numeric type that fits (Byte, Int16, or Single, a 32-bit float) instead of string.

SerializeYamlObject (Frontmatter/YamlSerializer.cs:135-136) then writes those numbers back out. As a result, every method that re-serializes the header changes the text of numeric-looking values: AddFrontmatter, ReplaceFrontmatter and CombineFrontmatter.

Before #174, every scalar was read as a string, so these values survived byte-for-byte. This is a regression.

Reproduction (at d8d1348)

string doc = "---\nversion: 1.10\nrelease: 1.0\nzip: 01234\nhex: 0x1F\nbig: 123456789012345678901234\nlat: 51.507351\nprice: 1234567.89\n---\nBody\n";
Frontmatter.AddFrontmatter(doc, new() { ["extra"] = "x" });

Output header:

version: 1.1
release: 1
zip: 1234
hex: 31
big: 1.2345679E+23
lat: 51.50735
price: 1234567.9
extra: x

ExtractFrontmatter(doc) reports these types:

CombineFrontmatter(doc, AsIs, AsIs, None) produces the same output, with no merging or renaming involved.

Why it matters

Adding one unrelated key silently rewrites values the caller never touched:

• Version strings change meaning: 1.10 becomes 1.1, and 1.0 becomes 1.
• IDs and zip codes lose their leading zeros.
• Coordinates and prices lose precision because they go through float.
• Large integers become floats in scientific notation.

Hand-written frontmatter usually leaves these values unquoted (for example Hugo weight: 1.10 or version: 2.0), so the "quote it" workaround from #137 does not help existing documents.

Suggested fix / acceptance criteria

• A re-serialized plain scalar produces the same text it was read from. Options:
  • Keep the original scalar text for numbers.
  • Parse only bool, null and int, and fall back to string whenever reformatting would change the text.
  • At minimum, parse floats as double/decimal and integers as long, and keep the raw string whenever value.ToString() differs from the source text.
• Round-trip tests through AddFrontmatter and CombineFrontmatter show each of these values unchanged: 1.10, 1.0, 01234, 0x1F, 51.507351, 1234567.89, 123456789012345678901234, .inf, +12.
• The Round-tripping strips quotes, so "true", "1.0", "007" and "null" come back as bool/number/null, and empty values become '' #137 tests in ScalarTypeRoundTripTests keep passing.