Fix stability, resource, and correctness bugs

[Thorium]

• HTML: decode hex charrefs (A) correctly; clamp table colspan/rowspan to spec limits (was OverflowException/OOM/hang on malicious spans); stack-safe HtmlNode.ToString (was uncatchable StackOverflow at ~5k nesting depth)
• HTTP: CombinedStream no longer treats partial reads as EOF (silent multipart truncation) and drops the 2GB int overflow; Range supports bytes=N-/-N; body streams disposed even when an upload faults
• CSV: skipRows survives re-enumeration of Rows; Save quotes headers, lone CR, and all document separators (round-trip corruption); Save(TextWriter) no longer disposes the caller-owned writer; reentrant enumeration synchronized
• JSON: NaN/inf/1e300 floats route through JsonValue.Float instead of throwing OverflowException; stray '/' no longer swallowed as whitespace
• Caching/IO: no Int32 overflow for >24.8-day expirations (could crash host); atomic temp-file-then-rename cache writes; conditional expiry removal; file-watcher subscriptions locked (IDE crash risk)
• WorldBank: HTTP-200 error bodies validated before caching (30-day cache poisoning); empty topics no longer throw KeyNotFoundException; URL segments escaped with EscapeDataString (path injection)
• XML: CreateRecord no longer corrupts structure for same-named nested paths
• DesignTime: Interlocked instance ids, locked dispose-action set, web cache key includes Encoding/row-limit parameters