Large bracketed pastes no longer flood the composer. On paste, if the text is
≥4 lines or >400 chars, insert a compact `[Pasted text #N +M lines]` chip and
hold the real content in a PasteStore; on submit, expand the chip back to the
full text before sending (free-code model). Single-pass String.replace keeps a
pasted block that itself contains a `[Pasted text #k]` literal safe.
The store is created ONCE in main.tsx and passed App→Composer (NOT per-composer)
so it survives the composer remounting on overlay open/close — a per-composer
store would lose a pending paste mid-compose. +6 unit tests (91 pass). Verified
live: paste 10 lines → chip; submit → transcript shows the full expanded code;
composer cleared.
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