fix: send only stdout delta in Bash terminal output to prevent replay

[william0wang]

Problem

Long-running Bash commands display duplicated output. The longer the command runs, the worse the duplication.

Root cause is a semantic mismatch:

• zcode's stdoutTail (progress events) and result content are cumulative snapshots — each carries the full tail of stdout so far.
• ACP's terminal_output.data is append semantics — the client writes each value as new bytes to the terminal.

dispatchTerminalUpdate passed the full snapshot into data every time, so N progress events replayed the output N times, and the final result event appended it once more.

Fix

Add terminalSentData: Map<callId, string> on ZcodeAcpServer to track the last-sent snapshot per Bash call. dispatchTerminalUpdate now:

1. Diffs the new snapshot against the last-sent one and emits only the suffix delta (full.slice(lastSent.length))
2. Falls back to full output when the snapshot isn't monotonic (tail rotation / prefix mismatch) to avoid data loss
3. Clears the callId entry on completed/failed

Test

• Added regression tests: repeated cumulative snapshots emit only deltas; result after progress emits only the unsent tail
• All 154 tests pass
  `

[gemini-code-assist]

Code Review

This pull request implements delta deduplication for terminal output updates by tracking the last-sent stdout snapshot per callId and emitting only the new suffix. This prevents long-running commands from replaying their entire output multiple times. The feedback highlights a critical edge case: when the stdout tail buffer rotates, the current prefix check fails and falls back to sending the full snapshot, causing output duplication. It is recommended to resolve this by finding the overlap between the consecutive snapshots and to add a corresponding regression test.

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[gemini-code-assist]

When the stdout tail buffer rotates (i.e., older output is discarded from the beginning of the cumulative snapshot), full.startsWith(lastSent) will be false. In this case, falling back to the full full snapshot causes the remaining overlapping part of the buffer to be replayed and duplicated in the terminal. Since long-running commands are exactly the ones that will trigger tail rotation, this defeats the purpose of preventing replay.

We can resolve this by finding the longest suffix of lastSent that matches a prefix of full (the overlap) and sending only the remaining suffix of full as the delta.

  let delta = full;
  if (lastSent) {
    if (full.startsWith(lastSent)) {
      delta = full.slice(lastSent.length);
    } else {
      const maxOverlap = Math.min(lastSent.length, full.length);
      for (let i = maxOverlap; i > 0; i--) {
        if (lastSent.endsWith(full.slice(0, i))) {
          delta = full.slice(i);
          break;
        }
      }
    }
  }