Refactor: HF-24 address prep-ultra polish (category, regex hoist, docs)

Address three nits surfaced by the final fresh-eyes review:

- ConfigParams.ts: stringifyCurrency was filed under @category 'Date and
  Time' (copied from sibling stringifyDateTime/Duration), placing a
  currency-formatting option under the Date and Time TypeDoc nav group.
  Move it to @category 'Number' to match where currencySymbol already
  lives, so currency-related config clusters together in generated API
  docs.

- ConfigParams.ts: expand the stringifyCurrency JSDoc to state plainly
  that the formatter calls the callback for every format string that
  reaches it, not only currency-shaped ones, and that returning undefined
  is the opt-out path for unsupported formats. IDE users reading the
  contract no longer have to infer this from the guide.

- format.ts: hoist the LCID-tagged currency-guard regex into a
  module-level const (LCID_CURRENCY_TAG) shared by
  defaultStringifyDateTime and defaultStringifyDuration. Avoids
  re-instantiating the same RegExp on every format() call (TEXT can be
  invoked thousands of times per recalc), and documents at the
  declaration site why the pattern is intentionally unanchored — Excel
  does not mix date/time tokens with currency tags in a single format
  string, so a mid-string match cannot misclassify a legitimate
  composite.

Author: marcin-kordas-hoc

src/ConfigParams.ts

@@ -314,15 +314,16 @@ export interface ConfigParams {
    * Sets a function that converts numeric values into currency-formatted strings.
    *
    * The function receives the raw value and the format string passed to `TEXT`
-   * and should return a string or `undefined`. Returning `undefined` lets the
-   * formatter fall through to the built-in number formatter, so a callback that
-   * recognizes only some format strings can safely opt out of the rest.
+   * and should return a string or `undefined`. The formatter calls this for
+   * every format string that reaches it, not only currency-shaped ones — return
+   * `undefined` for any format your callback does not handle and HyperFormula
+   * will fall through to the built-in number formatter.
    *
    * For more information, see the [Currency handling guide](/guide/currency-handling.md).
    *
    * @default defaultStringifyCurrency
    *
-   * @category Date and Time
+   * @category Number
    */
   stringifyCurrency: (value: number, currencyFormat: string) => Maybe<string>,
   /**

src/format/format.ts

@@ -10,6 +10,21 @@ import {RawScalarValue} from '../interpreter/InterpreterValue'
 import {Maybe} from '../Maybe'
 import {FormatToken, parseForDateTimeFormat, parseForNumberFormat, TokenType} from './parser'
 
+/**
+ * Detects Excel LCID-tagged currency tags (`[$SYMBOL-LCID]` with a non-empty
+ * SYMBOL portion). Shared by `defaultStringifyDateTime` and
+ * `defaultStringifyDuration` so a format string carrying such a tag short-
+ * circuits both date and duration dispatch and falls through to the
+ * number formatter (or the user-supplied `stringifyCurrency` callback).
+ *
+ * The pattern is intentionally unanchored: any occurrence of `[$SYMBOL-`
+ * in the format string triggers the guard. Excel does not mix date/time
+ * tokens with a currency tag in the same format string, so a mid-string
+ * match cannot misclassify a legitimate composite — every observed
+ * format string with a currency tag is currency-only.
+ */
+const LCID_CURRENCY_TAG = /\[\$[^\-\]]+-/
+
 export function format(value: number, formatArg: string, config: Config, dateHelper: DateTimeHelper): RawScalarValue {
   // Currency callback runs first so a user-supplied stringifyCurrency can
   // intercept LCID-tagged or bare-letter currency formats before the
@@ -100,7 +115,7 @@ export function defaultStringifyDuration(time: SimpleTime, formatArg: string): M
   // (H in CHF/HUF, m in AMD/HMD) that parseForDateTimeFormat would
   // otherwise interpret as time tokens. See defaultStringifyDateTime
   // for the symbol-vs-locale-modifier rationale.
-  if (/\[\$[^\-\]]+-/.test(formatArg)) {
+  if (LCID_CURRENCY_TAG.test(formatArg)) {
     return undefined
   }
   const expression = parseForDateTimeFormat(formatArg)
@@ -176,7 +191,7 @@ export function defaultStringifyDateTime(dateTime: SimpleDateTime, formatArg: st
   // and the `-` to distinguish currency tags (`[$USD-409]`, `[$€-2]`) from
   // Excel's locale-only modifier (`[$-409]`, `[$-F800]`), which is valid
   // on date/time formats and must continue to flow through this function.
-  if (/\[\$[^\-\]]+-/.test(formatArg)) {
+  if (LCID_CURRENCY_TAG.test(formatArg)) {
     return undefined
   }
   const expression = parseForDateTimeFormat(formatArg)