Reuse raw event structs with sync.Pool

[fanyang89]

Motivation

• Reduce per-event allocations when parsing bpftrace NDJSON/logfmt output by reusing event structs.
• Improve performance and GC pressure for high-throughput printf event streams.

Description

• Add a sync.Pool for raw event types and import sync in cmd_net.go, cmd_proc.go, cmd_mem.go, cmd_syscall.go, and cmd_vfs_raw.go.
• Reset pooled instances before parsing with *e = <Type>{} and unmarshal into the pooled object (e.g. logfmt.Unmarshal(buf, e)).
• Return pooled objects to the pool on both success and failure paths to avoid leaks, and call appendRow with the pooled pointer (e.g. err = appendRow(e)).
• Updated parsing functions: netJSONParseThenAppend, procJSONParseThenAppend, memJSONParseThenAppend, syscallJSONParseThenAppend, and both parse paths in cmd_vfs_raw.go to use the pools.

Testing

• Files were formatted with gofmt -w to ensure idiomatic formatting.
• No automated unit tests were executed as part of this change.

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Codex Task