jq comments (#) are rejected in all positions, and the limitation is undocumented

[gh123man]

Description

fastjq rejects jq comments (#) in every position. jq 1.8.1 and gojq both accept them.

This isn't listed under Limitations in the README, and docs/SYNTAX.md doesn't mention
comments at all — so a caller migrating a working jq program has no warning. It bites
hardest when queries are authored by humans in a config file or UI, where a trailing
# why this filter exists is completely ordinary.

Reproducer

package main

import (
	"fmt"

	"github.com/DataDog/fastjq"
)

func main() {
	for _, q := range []string{
		`.foo = "baz" # set foo`,
		"# set foo\n.foo = \"baz\"",
		".foo = \"baz\"\n# trailing note",
		".a # mid\n| .b",
	} {
		_, err := fastjq.Compile(q)
		fmt.Printf("%-28q -> %v\n", q, err)
	}
}

Expected output: all four compile; comments are stripped as whitespace.

$ echo '{}' | jq '.foo = "baz" # set foo'
{
  "foo": "baz"
}

Actual output:

".foo = \"baz\" # set foo"    -> unexpected trailing input: "# set foo"
"# set foo\n.foo = \"baz\""   -> unexpected character "#"
".foo = \"baz\"\n# trailing note" -> unexpected trailing input: "# trailing note"
".a # mid\n| .b"              -> unexpected trailing input: "# mid\n| .b"

Suggested fix

Comment skipping belongs in the scanner's whitespace handling: on #, consume to the next
\n or end of input. jq's rule is that # starts a comment except inside a string literal,
and a backslash-newline continuation does not extend a comment (jq 1.7+ treats \ at end
of a comment line literally). The common cases are the four above.

If comments are out of scope by design, listing them under Limitations next to the
regex/decnum notes would be enough to save the next migrator the debugging.

Environment

• fastjq version: c3336f252fa23bbda9a611024f6398904330cd9a (master, untagged)
• Go version: go version go1.25.4 darwin/arm64
• OS / arch: macOS, arm64