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Neil
ed881889c4
`extractPartialEscapeTail` broke its own fold invariant
(extract(a + b) === extract(extract(a) + b)) in the oscEsc/stringEsc states,
so a PTY read that split there produced a different pending tail than the
same bytes delivered whole — the tail snapshots append after a restore.
Two causes, both in the "ESC did not terminate the string" branch:
- CAN/SUB were routed through `stateAfterEscByte`, which maps them back to
`esc` instead of aborting to ground. `extractPartialEscapeTail('\x1bPx\x1b\x18X0abc')`
returned '\x1b\x18X0abc'; the chunk-split fold returned ''.
- A second ESC opened its new sequence at `i - 1` rather than at itself.
`extractPartialEscapeTail('\x1b] \x1b\x1b^')` returned '\x1b\x1b^' whole but
'\x1b^' folded. The fold was right — xterm starts the sequence at the second ESC.
The existing fuzz only asserted the fold as `advance(extract(pending), chunk)`,
which is a tautology because every PENDINGS entry is already a tail. Replaced
with a sweep that re-splits the combined stream at every code-unit boundary,
and extended the alphabet (NUL, 0x20 intermediate, CJK) and SEQUENCES with
CAN/SUB and doubled-ESC-inside-string cases. A 1.25M-split fold fuzz over a
VT alphabet goes from 421 failures to 0.