refactor(encode): scope URLSearchParams sentinel to encode module

REUSED_SEARCH_PARAMS is only used by encodeQualifierParam. Moving it
from src/constants.ts into src/encode.ts keeps the mutable singleton
private to its sole consumer so its side effects can't leak to other
modules.

Author: jdalton

src/constants.ts

@@ -1,22 +1,7 @@
 /**
  * @fileoverview Shared constants used across the PURL library.
- * Includes loop sentinels and reusable URL search parameter utilities.
  */
 
-import { URLSearchParamsCtor } from './primordials.js'
-
 const LOOP_SENTINEL = 1_000_000
 
-const REUSED_SEARCH_PARAMS = new URLSearchParamsCtor()
-
-const REUSED_SEARCH_PARAMS_KEY = '_'
-
-// '_='.length
-const REUSED_SEARCH_PARAMS_OFFSET = 2
-
-export {
-  LOOP_SENTINEL,
-  REUSED_SEARCH_PARAMS,
-  REUSED_SEARCH_PARAMS_KEY,
-  REUSED_SEARCH_PARAMS_OFFSET,
-}
+export { LOOP_SENTINEL }

src/encode.ts

@@ -2,11 +2,6 @@
  * @fileoverview URL encoding functions for PURL components.
  * Provides special handling for names, namespaces, versions, qualifiers, and subpaths.
  */
-import {
-  REUSED_SEARCH_PARAMS,
-  REUSED_SEARCH_PARAMS_KEY,
-  REUSED_SEARCH_PARAMS_OFFSET,
-} from './constants.js'
 import { isObject } from './objects.js'
 import {
   ArrayPrototypeToSorted,
@@ -18,6 +13,13 @@ import {
 } from './primordials.js'
 import { isNonEmptyString } from './strings.js'
 
+// Module-private reusable URLSearchParams for encodeQualifierParam. Kept
+// private here so mutation side-effects can't leak to other modules.
+const REUSED_SEARCH_PARAMS = new URLSearchParamsCtor()
+const REUSED_SEARCH_PARAMS_KEY = '_'
+// '_='.length
+const REUSED_SEARCH_PARAMS_OFFSET = 2
+
 /**
  * Encode package name component for URL.
  */