Intl: canonicalize physical string storage before locale diagnostics

Parent #473; related #475 and discovered during #825 validation. Exact pushed baseline 8e1e441 aborts in the ReleaseSafe test262 Intl subtree: StringCell reports a truncated WTF-8 sequence while constructing RangeError text for an invalid locale. The captured bytes end in the Latin-1 byte df for an en-ß input. canonicalizeLocaleList currently passes v.asStr() directly for a string input, while canonicalizeLocaleTagForIntl interpolates those bytes into an engine string. The physical Latin-1 representation must be converted through the existing StringData/WTF-8 boundary, not sanitized repaired text substituted or the assertion disabled. Preserve the original invalid locale and RangeError, add Latin-1/UTF-16/WTF-8 and lone-surrogate cases, validate individual and list inputs and supportedLocalesOf/constructor entry points, then rerun the full scored Intl corpus. The complete baseline abort transcript and stack sample are retained locally; no completed Intl denominator or regression claim is made from that aborted run.

Landed and pushed as 3178a6b0 — fix(intl): canonicalize locale string storage (#829).

The exact baseline reproducer, Intl.NumberFormat.supportedLocalesOf("en-ß"), aborted the process with exit 134. The committed implementation reads direct strings and coerced list elements through the canonical WTF-8 boundary; direct, array, and object-coercion forms now produce the required RangeError with the intact engine-visible message.

Verification:

• full test/intl402: baseline aborted before a denominator; committed candidate completes at 3,341 pass / 0 fail
• focused Debug: 1 passed, 0 failed/leaked
• unsuppressed ReleaseSafe TSan: 1 passed, 0 failed/leaked/races

CI for the pushed commit remains the closure gate.

Terminal descendant-main verification is now complete: CI run 33312141595 on f4f00803 completed successfully with all 56 jobs terminal and no failed/cancelled jobs. This run contains the landed #829 implementation and the subsequent integration history. Closing the issue as completed; any newly reproduced defect should retain its own exact witness rather than reopening this verified scope.