Fix blocking mux reads and RAW download parsing

[durck]

Summary

• Keep bufferedConn as a prefix replay wrapper and avoid reading from the underlying connection while buffered prefix data is still available.
• Add a regression test that uses a 4096-byte caller buffer with a blocking underlying connection.
• Read RAW download requests with bounded protocol parsing instead of relying on a single TCP read.
• Cover .sh, .ps1, raw/binary filenames, split TCP chunks, empty names, and over-length names.

Validation

• go test -count=1 ./pkg/mux ./internal/server/tcp

Note: local go test ./... is not clean on current upstream/main with Go 1.25.5 before this branch is applied; it fails on existing non-constant fmt.Errorf vet errors in server packages and internal/terminal/TestQuoteHandling.

[NHAS]

Hi there

Why was this deleted?

[durck]

Sorry about that. I accidentally deleted the source branch while cleaning up stale branches in my fork.

The change is still useful, so I recreated it as #221 with the same scope and validation: #221