Avoid redundant package resolution

[renkun-ken]

Summary

• remember each document's last successful ordered package request
• normalize away vector names/encoding metadata before comparing requests
• avoid restarting package resolution after transient parse errors
• preserve package order so masking and namespace precedence remain correct

Fixes #754.

Testing

• Rscript -e 'devtools::test(filter = "document-core|handlers-textsync|workspace-core|completion", reporter = "summary")'
• git diff --check
• changed-file linting

The full local suite was also run. It reached existing environment-related failures in diagnostics/formatting subprocess tests (gzfile(...): cannot open the connection), while the focused package-resolution and dependent tests passed.