streaming.WriteCompaction packages a 'compaction' event with the
kept/older turn counts, summary tokens and provider-reported
window/used tokens so the client can hint 'context optimized' to the
user without parsing the stream body.
streamChat runs Compact before BuildMessages and writes the event
right after start, ensuring the client sees it before any chunk is
emitted. Add a runner test that exercises limitRAGContext to keep
the system prompt + RAG block under the configured window.
Runner.Compact folds the older portion of history into a single
system-role summary when the previous turn's input tokens cross
threshold_ratio × MaxContextWindow. When summarization fails, the
runner falls back to truncateToBudget so a flaky summarize call never
breaks the user's request.
EstimatePromptTokens / totalPromptTokens give a conservative count
(roughly 3 chars per token) used by both the compaction trigger and
BuildMessages' new limitRAGContext / fitHistory helpers to cap the
prompt inside the provider's reported window before the request goes
out. Covers the first-turn case where no usage has been reported yet.
Adds runner_compaction_test.go with table-driven coverage for the
disabled, below-threshold, short-history, unknown-window, fallback
and first-stream cases, plus a regression for the RAG-context trimmer.
