finish() derived the exact omitted count from the pre-trim prefix/suffix
budget, but the UTF-8 boundary trims drop additional partial-codepoint bytes,
so an "Omitted N bytes" notice overstated what was kept (head maxBytes:2 over
'a€b' returned 'a' but claimed exact 3 when 4 bytes are absent). Derive the
count from the bytes actually returned (total − keptPrefix − keptSuffix) so
exact metadata matches the text and atLeast stays a valid lower bound.
When headTail budgets cover the whole stream, omitted is 0 and the two
retained halves are contiguous — the split is artificial and a multibyte
codepoint can straddle it. finish() now decodes the contiguous buffer as one
in that case; the per-side UTF-8 boundary trims and separate decoding apply
only when a real middle gap exists. Without this, a headTail retainer could
drop a character while reporting truncated:false.
Ship @deepseek-ai/dsh-retention under packages/util/: pure ItemRetainer /
TextRetainer plus neutral notice helpers, so tools that cap model-facing
output share one "what did we keep, what did we omit, may we stop reading"
mechanic while keeping grouping, exit codes, provider errors, and recovery
prose tool-owned. The two retainers are separate names because they differ in
resource model: item-head can stop the upstream on the first over-cap probe
(shouldStop), while text tail/head-tail must read to the end. The library
documents glob/grep/bash/web_fetch/web_search mappings but migrates no tool
yet — glob/grep don't exist, and migration is deliberately separate work.
Flips the RFC to implemented/ and rewrites its skeleton to shipped reality.
Dudu-0223