refactor(ime): replace manual wire assembly with MessageBuilder/push_msg and add structured logging

[kkimdev]

This is PR 1 of a stacked series (split from #27 for reviewability). It is a behavior-neutral refactor with zero protocol changes.

Every instance of the open-coded Wayland wire-format assembly pattern:

let mut msg = Vec::new();
msg.extend_from_slice(&sender_id.to_ne_bytes());
let len = (builder.payload.len() + 8) as u32;
let word2 = (len << 16) | opcode;
msg.extend_from_slice(&word2.to_ne_bytes());
msg.extend_from_slice(&builder.payload);
queue.push((msg, Vec::new()));

is replaced with a single call:

push_msg(&mut queue, sender_id, opcode, builder);

MessageBuilder::build_message produces byte-identical output, so this is behavior-neutral.

Also adds structured log::trace/debug/info instrumentation to every IME-related handler in both text_input.rs and keyboard.rs so the host↔guest event flow is observable in logs.

Next in the stack

• PR 2: Core IME correctness — serial propagation, set_preedit_region/confirm_preedit handlers, commit_state fix, activation state machine
• PR 3: Korean IME backspace hold heuristic

[kkimdev]

Split from #27 to make review manageable. This PR is the behavior-neutral foundation (MessageBuilder refactor + logging). The follow-up PRs (core IME correctness, then the backspace heuristic) are being prepared and will each link back here.