The @as error message omits float and bigint, which @as accepts #8973

Found by differential semantic drift analysis from a7721303 to b123b7c3.

What is broken

The @as error message omits float and bigint, which @as accepts.

• ast_untagged_variants.ml L27-L30: "A variant case annotation @as(...) must be a string, integer, boolean, null, or undefined."
• runtime_tag_of_parsetree (L104-L113) also accepts Pct_float and Pct_bigint: type t = | @as(1.5) A | @as(12n) B compiles to [1.5, 12n], and | @as(foo) C gets the message above.
• compiler/ml/UntaggedVariants.md L65 (Document the rules of untagged variants, with tests pinning them #8952) lists float and bigint and then quotes the message (L74-L75).

Minor, same document: the rule 6 table (L143) gives "At most one case can be a C type." where the code prints "an array or tuple type" for arrays and tuples (L47-L48).

Possible cause (speculation only)

A guess: float and bigint tags were added without updating the message, and #8952 documented the accepted set beside it.