Fix diff renderer dropping repeated style after :reset op

[reisub]

filter_redundant_style tracks last_style so it can dedupe :style ops that match the previous one. But it ignored :reset ops, which write \e[0m to the terminal and clear all SGR state. When two adjacent row spans share a style, the sequence was:

1. row N: :reset, :move, :style(X), :text(...) -- updates last_style to X
2. row N+1: :reset, :move, :style(X), :text(...) -- :reset wipes terminal SGR state -- :style(X) matches last_style and is deduped away

So row N+1 emitted only text, leaving its cells rendered without any styling. Symptom: multi-row styled spans (e.g. a coloured-band "user message" panel, or a styled status bar that shares a style with a preceding indicator) render styled on the first row and bare on every row after.

Fix: add a clause to filter_redundant_style for :reset that clears the tracked last_style, forcing the next :style op to be emitted in full. Regression test included.