fix: codebase-wide bug sweep

[kacy]

Summary

• fix hpack integer decode: u6 shift variable wraps at 63, defeating the overflow guard — use u8 with explicit @intCast for the shift operand
• fix raft snapshot install unconditionally regressing commit_index and last_applied — use @max to preserve monotonicity
• fix raft replication advancing commit_index before the commit action is successfully queued — swap order so index only advances on success
• fix hpack literal header encoding truncating values >127 bytes via unchecked @intCast to u8 — add length bounds checks
• fix http/2 header block padding underflow when padded_len > payload.len — add bounds validation before subtraction
• fix http content-length parsing returning 0 on malformed values (e.g. "abc"), masking bad requests as empty bodies — return BadRequest error instead
• fix leadership transfer reporting success when the become_follower action fails to queue — return false on failure
• fix gossip suspect timeout subtraction wrapping on tick counter overflow — add monotonicity guard
• fix json special characters (quotes, backslashes, newlines) unescaped in http error response bodies
• fix @bitCast of signed f_bsize to u64 in disk space check — use validated @intCast
• fix s3 delete returning 204 success when the underlying delete operation fails — return 500
• fix silent setsockopt failures when setting socket timeouts — log warnings

13 commits, 12 bug fixes + fmt.