fix(codemode): honor Uint8Array.from mappers, compare byte searches without coercion, skip flatMap holes, and truncate fractional radixes - #57285

Problem

Four reproduced Code Mode issues where a built-in silently returned a different value from JavaScript. The worst case: the usual browser base64 decode, Uint8Array.from(atob(b64), (c) => c.charCodeAt(0)), returned all zeros, because Uint8Array.from ignored its mapper.

Closes #57244, #56924, #56925, #51807.

Change

 Uint8Array.from(atob("aGk="), (c) => c.charCodeAt(0))
-[0, 0]                   mapper and thisArg ignored; a non-callable mapper accepted
+[104, 105]               mapper sees the original values after the source is consumed, as TypedArray.from does

 new Uint8Array([0, 1, 2]).includes("1")      // also indexOf(true), lastIndexOf(null)
-true                     search value coerced to a number
+false                    non-numbers never match a byte; start indexes still coerce

 [1].flatMap(() => [, 2])
-[undefined, 2]           spread turned holes into elements
+[2]                      holes skipped; explicit undefined kept

 (35).toString(36.9)
-RangeError               range checked before the radix was truncated
+"z"                      our check removed; the host truncates, then throws RangeError for 0, 37, NaN, ...

src/stdlib/bytes.ts   collectBytes   takes an optional mapper/thisArg; collects raw values, maps, then converts
                      from           passes args[1], args[2] through
                      indexOf, lastIndexOf, includes   search value used as-is when it is a number, else NaN
src/stdlib/array.ts   flatMap        forEach over the mapped items, which skips holes
src/stdlib/number.ts  toString       radix range check removed

Credit: the byte-search, flatMap, and radix fixes are #56926 (@sdivyanshu90), #56927 (@sdivyanshu90), and #51808 (@icearia0219), unchanged except a comment. The Uint8Array.from fix follows #57257 (@r266-tech) and its tests, but reuses collectBytes instead of a second iteration loop (8 source lines instead of 30). All three are co-authors on the commit; those PRs can close once this merges.

Out of scope

• Uint8Array.from still rejects array-likes ({ length: 2, 0: 7 }), as before.
• The harder Code Mode issues, codemode: concurrent async functions lose updates to shared variables #54274 (concurrent async functions lose updates) and codemode: execute hangs until eviction when a program awaits a promise nothing can settle #54340 (an await nothing can settle hangs, since core sets no timeout), have their own PRs (fix(codemode): run each job to completion #54276, fix(codemode): fail a program that stalls instead of holding the turn #54342) and need separate review.

Verification