fix(parsing): resolve a non-specific '!' tagged scalar as a string

The non-specific '!' tag resolves a node by kind alone, with no implicit
typing, so '! 12' is the string "12", not the integer 12. The event
stream already carried the tag correctly, but typed-value construction
fell through to plain-scalar resolution and produced an int.

Add the '!' case to tagged-scalar composition so it yields a string.
Surpasses libyaml on S4JQ (the last parse-ok gap); whole-suite
conformance 391/402, canonical events 308/308.

CHANGELOG.md

@@ -22,6 +22,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Resolve a scalar with the non-specific `!` tag as a string. The `!` tag forces
+  resolution by kind alone, with no implicit typing, so `! 12` is the string "12"
+  rather than the integer 12. The typed value now honours it (the event stream
+  already did). Surpasses libyaml on S4JQ.
 - Reject a bare `-` before a flow indicator inside a flow collection. Per
   `ns-plain-first` a `-` begins a plain scalar in flow only when a flow-safe
   character follows; `[-]` and `[-, -]` are malformed, not a sequence of `-`

Sources/Parsing/Parser.ScalarComposition.swift

@@ -28,6 +28,10 @@ extension PureYAML.Parsing.Parser {
             return nil
         }
         switch tag {
+        case "!":
+            // The non-specific `!` tag resolves a scalar by kind alone: a scalar is
+            // a string, with no implicit typing, so `! 12` is "12" not the int 12.
+            return .string(value)
         case "tag:yaml.org,2002:str":
             return .string(value)
         case "tag:yaml.org,2002:int":

Tests/Fixtures/yaml-test-suite-manifest.yaml

@@ -10,8 +10,8 @@ source:
 # oracle:   json if in.json present, else none
 # status:   passing (PureYAML conformant today) | gap (not yet conformant)
 total: 402
-passing: 390
-gap: 12
+passing: 391
+gap: 11
 cases:
   - id: "229Q"
     name: "Spec Example 2.4. Sequence of Mappings"
@@ -2626,7 +2626,7 @@ cases:
     localPath: yaml-test-suite/S4JQ/in.yaml
     expected: parse-ok
     oracle: json
-    status: gap
+    status: passing
     byteCount: 55
     lineCount: 4
   - id: "S4T7"