JSON string fragments accumulated across chunks, reassembled by
parse_args once the payload looks complete. Mutually exclusive with
args (see __post_init__ and ingest).
Append to this list or rebind it wholesale; do not mutate entries in place.
The _args_* fields hold a JSON lexer folded over the fragments scanned so
far, and _sync_args_fragment_state detects a rebind or a truncation but
cannot detect an edit that leaves the list's identity and length unchanged
(e.g. pop() then append()), which would leave the scan stale and yield
another payload's args. Detecting that would cost a per-chunk rescan, which
is the quadratic behavior this state exists to remove.
