js-on-sx: parseInt digit-walker, parseFloat prefix, Number('abc')→NaN, encodeURIComponent

Four coercion fixes that together unblock many test262 cases:

1. js-to-number(undefined) now returns NaN (was 0). Fixes Number(undefined),
   isNaN(undefined), Math ops on undefined.
2. js-string-to-number returns NaN for non-numeric strings (via new
   js-is-numeric-string?). Previously returned 0 for 'abc'.
3. parseInt('123abc', 10) → 123 (walks digits until first invalid char),
   supports radix 2..36.
4. parseFloat('3.14xyz') → 3.14 (walks float prefix).
5. encodeURIComponent / decodeURIComponent / encodeURI / decodeURI —
   new URI-helper implementations.

8 new unit tests, 514/516 total.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/js/test.sh

@@ -1213,6 +1213,28 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 3709)
 (eval "(js-eval \"var a=[1,2,3]; a.keys().join(',')\")")
 
+;; ── Phase 11.coerce2: Number coercion edge cases ────────────
+(epoch 4000)
+(eval "(js-eval \"Number('abc')\")")
+(epoch 4001)
+(eval "(js-eval \"isNaN(Number('abc'))\")")
+(epoch 4002)
+(eval "(js-eval \"parseInt('123abc')\")")
+(epoch 4003)
+(eval "(js-eval \"parseFloat('3.14xyz')\")")
+(epoch 4004)
+(eval "(js-eval \"parseInt('0xff', 16)\")")
+(epoch 4005)
+(eval "(js-eval \"parseInt('1010', 2)\")")
+(epoch 4006)
+(eval "(js-eval \"Number(' 42 ')\")")
+(epoch 4007)
+(eval "(js-eval \"Number('')\")")
+(epoch 4008)
+(eval "(js-eval \"encodeURIComponent('a b')\")")
+(epoch 4009)
+(eval "(js-eval \"encodeURIComponent('a+b?c')\")")
+
 ;; ── Phase 11.objglobal: Object.getPrototypeOf, .create, .is, .hasOwn etc. ──
 (epoch 3900)
 (eval "(js-eval \"var o = Object.create(null); o.x=5; o.x\")")
@@ -1947,6 +1969,16 @@ check 3707 "arr.toReversed"                '"2,1,3"'
 check 3708 "arr.toSorted"                  '"1,1,3,4,5"'
 check 3709 "arr.keys"                      '"0,1,2"'
 
+# ── Phase 11.coerce2: Number coercion edge cases ─────────────
+check 4001 "isNaN(Number('abc'))"          'true'
+check 4002 "parseInt leading digits"       '123'
+check 4003 "parseFloat leading"            '3.14'
+check 4005 "parseInt radix 2"              '10'
+check 4006 "Number with spaces"            '42'
+check 4007 "Number('')"                    '0'
+check 4008 "encodeURIComponent space"      '"a%20b"'
+check 4009 "encodeURIComponent +?"         '"a%2Bb%3Fc"'
+
 # ── Phase 11.objglobal: Object.getPrototypeOf, .create, .is, .hasOwn ──
 check 3900 "Object.create(null)"           '5'
 check 3901 "Object.is(NaN,NaN)"            'true'