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/memory.rs

@@ -108,6 +108,23 @@ pub const SYSVAR_HLD: u32 = SYSVAR_BASE + 28;
 pub const SYSVAR_LEAVE_FLAG: u32 = SYSVAR_BASE + 32;
 /// Throw code left by a compiled stack-guard fault for `_STACK_FAULT_`.
 pub const SYSVAR_FAULT_CODE: u32 = SYSVAR_BASE + 36;
+/// DPL: digits right of the rightmost punctuation in the last converted
+/// number; negative when the token carried no punctuation.
+pub const SYSVAR_DPL: u32 = SYSVAR_BASE + 40;
+/// NH: high-order cell of the last single-cell conversion, so an
+/// out-of-range token can be recovered as a double.
+pub const SYSVAR_NH: u32 = SYSVAR_BASE + 44;
+
+/// Seed for [`SYSVAR_DPL`] before conversion starts.
+///
+/// `SwiftForth` seeds DPL with a negative value and bumps it once per digit,
+/// so an unpunctuated token still ends up negative. Punctuation resets the
+/// counter to zero, which makes the final value the digit count right of the
+/// rightmost punctuation character.
+///
+/// The exact seed is observable: `sf64` reports DPL as -1020 after `1234`
+/// and -1023 after `-1`, both of which pin it to -1024.
+pub const DPL_INIT: i32 = -1024;
 
 #[cfg(test)]
 mod tests {
@@ -149,6 +166,9 @@ mod tests {
             SYSVAR_NUM_TIB,
             SYSVAR_HLD,
             SYSVAR_LEAVE_FLAG,
+            SYSVAR_FAULT_CODE,
+            SYSVAR_DPL,
+            SYSVAR_NH,
         ];
         for offset in all_offsets {
             assert!(offset + CELL_SIZE <= SYSVAR_BASE + SYSVAR_SIZE);