rework Silver string building and add node:net client shims

• replace deep rope-heavy string appends with a builder-backed concat path
• teach the VM, compiler, JIT glue, and GC to read, flush, and preserve builder-backed strings
• keep arguments/object slot syncing and related frame semantics aligned with the new slot behavior
• finish fetch-backed node:http/node:https client shims and expose node:url.parse
• add regression tests for string append loops, HTTP(S) shims, and node:url exports