feat(core): SwiftForth input number conversion, DPL and NH

Punctuation (`,` `.` `+` `/` `:` and an embedded `-`) after the leftmost
digit now forces double-cell conversion, so `12.34`, `1,234`, `12:30:45`
and `2026-08-06` convert as doubles. Only a trailing `.` worked before,
and `1.5` was an "unknown word" error.

The punctuation is a double-cell marker, not a fractional point, so the
scale has to travel separately: DPL carries the digit count right of the
rightmost punctuation character (negative when there was none), which is
what lets `<# #>` place the point back on output. NH carries the high
cell a single-cell conversion drops, so a token that overflows a cell is
still recoverable as a double.

parse_number and parse_double_number duplicated the prefix and sign
handling and could not share a DPL counter, so they collapse into one
parse_numeric_literal that reports which kind it converted.

Verified token-for-token against sf64. One deliberate divergence: WAFER
keeps accepting a sign before a base prefix (`-$FF`), which sf64 rejects.

crates/core/src/wordhelp.rs

@@ -350,6 +350,16 @@ pub const WORD_DOCS: &[(&str, &str, &str)] = &[
         "Convert digits, accumulating into ud.",
     ),
     ("BASE", "( -- addr )", "Variable holding the number base."),
+    (
+        "DPL",
+        "( -- addr )",
+        "Variable: digits right of the last punctuation; negative if none.",
+    ),
+    (
+        "NH",
+        "( -- addr )",
+        "Variable: high cell dropped by the last single-cell conversion.",
+    ),
     ("HEX", "( -- )", "Set BASE to sixteen."),
     ("DECIMAL", "( -- )", "Set BASE to ten."),
     // -- Core: strings --