[FOUND-113] (2/5) Add buffered socket read helpers

[mattwd7]

Summary

• Add buffered text-read helpers to socket instance methods (read_line, read_from_buffer, clear_read_buffer).
• Preserve over-read bytes for mixed line/body parsing workflows.
• Keep change additive without modifying existing binary call paths.

Test plan

• Meta/unit and integration tests pass in stacked head branch
• No behavioral change expected for existing binary protocol path

[mattwd7]

read_line and the @read_buffer are needed by the meta protocol specifically.

Binary protocol responses are length-framed:

• fixed 24-byte header
• header tells you exact body size
• client then reads exactly that many bytes

Because binary always does fixed-size reads in sequence, it doesn’t need a cross-call spill buffer.
Meta protocol responses are mixed:

• first read a line (e.g. VA 5 f0 ...\r\n)
• then read a known number of body bytes (5)
• then consume trailing \r\n

A single socket read can pull more than just the line, so leftover bytes must be kept for the next body read. That’s what @read_buffer is for.

[mattwd7]

Self-reviewed ✅