Fix off-by-one in RPC result attribution

[plan9better]

Problem

configure() prepends "configure terminal" to the payload, so the device returns N+1 results for an N-line patch. summary() zipped those results against the patch commands starting at index 0, so every command was reported with the previous command's result.

Two visible consequences:

• Failures are blamed on the following command.
• The first real command absorbs the prologue's empty result and always logs msg="ok", so a failed command can be reported as successful.

zip() silently truncates on the length mismatch, which is why this never surfaced as an error.

Impact

Real log from a failing run against a switch, with a 14-line patch:

result cmd="no channel-group 3"                msg="ok"
result cmd="exit"  error_code=-3004  "the string does not match any command in current mode"
result cmd="port-channel 3 lacp-mode dynamic"  msg="ok"
result cmd="interface eth-0-3"  error_code=-1000  "% Can't set lacp-mode, agg3 have existed"

This reads as nonsense: exit cannot produce -3004 in interface mode, and interface eth-0-3 cannot produce a lacp-mode error. Shift by one and it resolves exactly:

reported against	actually belongs to
exit	no channel-group 3 (invalid syntax — the real bug, in the patch)
interface eth-0-3	port-channel 3 lacp-mode dynamic
exit	mlag 3

The misattribution actively obscured the root cause: no channel-group 3 was logged msg="ok" while it was in fact the command that failed and stalled the whole patch.

Fix

• configure() drops the prologue's result entry, so what it returns aligns 1:1 with commands. The prepend stays an implementation detail of the function that introduced it.
• summary() bails on a length mismatch instead of letting zip() truncate silently, so this class of misalignment fails loudly.

Testing

cargo build clean. Pre-existing cargo fmt diff and 3 clippy warnings are untouched on master and left alone to keep the diff reviewable.

Verified by hand against the log above: with the prologue dropped, all 14 results line up with their commands and each error lands on a command that can actually produce it.

🤖 Generated with Claude Code