🐛 improve pack string overflow errors

[rauhryan]

Summary

• preflight packed string size before copying into the transfer buffer
• throw descriptive RangeError for oversized text/element-id payloads instead of leaking raw TypedArray offset is out of bounds
• document the transfer-buffer overflow error contract in the renderer spec
• add pack tests for oversized text content and element ids

Test

• deno test

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/clayterm@33

commit: a62376d