Strings that YAML 1.1 reads as booleans or numbers are written unquoted ("NO" → NO, which Jekyll/PyYAML read as false; "12:30" → 750) #188

What's wrong

The serializer in Frontmatter/YamlSerializer.cs:36-40 is built with .WithQuotingNecessaryStrings() and no argument. That quotes only strings that are ambiguous under the YAML 1.2 core schema (true, null, 1.0, 007, and so on). It does not quote strings that are booleans or numbers under YAML 1.1:

• yes, no, on, off, y, n, in any case
• sexagesimal values such as 12:30
• 1_000
• 0b101

Every write path goes through this serializer: CombineFrontmatter (Frontmatter.cs:113), AddFrontmatter (:200), ReplaceFrontmatter (:222) and SerializeFrontmatter (:265). A string value with one of these spellings therefore comes out as a bare scalar.

#137 set out to keep strings from changing type for "Jekyll, Hugo, Astro, js-yaml, PyYAML". Jekyll (Ruby Psych) and PyYAML are YAML 1.1 parsers, so these values still change type for them.

Reproduction (at 23c87b1)

Caller-supplied values:

Frontmatter.AddFrontmatter("Body", new() { ["country"] = "NO", ["enabled"] = "yes", ["duration"] = "1:30" });

---
country: NO
enabled: yes
duration: 1:30
---
Body

Round trip of values that were quoted in the source:

Frontmatter.CombineFrontmatter(
    "---\nk0: \"yes\"\nk1: \"No\"\nk2: \"on\"\nk3: \"OFF\"\nk6: \"12:30\"\nk8: \"1_000\"\nk9: \"0b101\"\n---\nBody\n",
    FrontmatterNaming.AsIs, FrontmatterOrder.AsIs, FrontmatterMergeStrategy.None);

All seven values come back unquoted: k0: yes, k1: No, k2: on, k3: OFF, k6: 12:30, k8: 1_000, k9: 0b101. In the same run, strings that are ambiguous under YAML 1.2 ("01234", "1e3", "TRUE", "0x1F", ".inf", "~") do stay quoted.

How YAML 1.1 consumers read that output:

Why it matters

Static-site generators and scripts consume the frontmatter this library writes. A caller who writes the string "NO" (a country code), "yes" or "no" (an answer field), or "1:30" (a duration) gets a boolean or an integer downstream. This is the "Norway problem". A formatting pass also silently changes the type of values whose authors quoted them specifically to prevent this.

Suggested fix / acceptance criteria

• Build the serializer with .WithQuotingNecessaryStrings(quoteYaml1_1Strings: true). YamlDotNet 18.1.0 has this overload. With it, the output is a: "yes", b: "NO", c: "12:30", d: "1_000", e: "0b101", while the bool true and the string hello stay bare.
• Watch the interaction with the read path. WithAttemptingUnquotedStringTypeDeserialization reads a plain published: yes as the string "yes". With the flag on, that value would be rewritten as published: "yes", which changes its meaning for YAML 1.1 readers in the opposite direction.
• For scalars that come from the document, emitting each one with its original style (part C of Redesign CombineFrontmatter name matching, merging and YAML rewriting (fixes #175 #176 #177 #178 #162 #167 #154 #150 #151 #153 #161) #180) is the real fix.
• The flag is still needed for caller-supplied values, which Redesign CombineFrontmatter name matching, merging and YAML rewriting (fixes #175 #176 #177 #178 #162 #167 #154 #150 #151 #153 #161) #180 routes through the existing Serializer. Either land the flag with or after Redesign CombineFrontmatter name matching, merging and YAML rewriting (fixes #175 #176 #177 #178 #162 #167 #154 #150 #151 #153 #161) #180 step 1, or apply it only on the caller-value path.
• Tests:
  • AddFrontmatter with the string values "NO", "yes", "on", "Off", "y", "12:30", "1_000" and "0b101" writes each one quoted.
  • A round trip of quoted source values through CombineFrontmatter(AsIs, AsIs, None) keeps them quoted.
  • A plain draft: false stays a bare boolean.
  • The existing ScalarTypeRoundTripTests still pass.

Related: #137 (closed; covered the YAML 1.2 cases), #180 (open; preserving document scalar style).