Recompute restated-suffix usage after compaction

The kept-verbatim suffix is no longer copied with stale usage. After
compaction the conversation begins [summary(user), kept_user,
kept_assistant, ...]; we rewrite each kept assistant's usage so the window
reads as a fresh conversation anchored at the summary.

- runSingleCompactionTurn returns the summary text plus its size (the
  provider-reported output token count for the summary turn, falling back
  to the word-count heuristic when usage is absent).
- rewriteWithSummary takes summary_size and forward-walks the kept suffix
  maintaining a synthetic cumulative input: non-assistant messages add
  their messageTokenEstimate; each assistant gets input = running
  cumulative total, with the full prompt (input+cache_read+cache_write
  worth) collapsed into input and cache buckets zeroed (a rewrite busts the
  provider prefix cache). output/reasoning are copied verbatim; assistants
  without prior usage stay null.

This keeps the cumulative-delta sizing (computeSplit/turnTokenEstimate) and
any future TUI context-window readout correct without special-casing
compaction. Tests cover the single- and multi-turn restated chains
(including cache-bucket collapse).

Test stub ScriptedTurn gains an optional usage stamp.

0 files changed, 0 insertions, 0 deletions