Slim rework of despotak's modified-keypad fix in #97290. Mirror existing
non-keypad mappings for modified keypad keys, including lock-state variants,
so Alt+keypad Enter reaches the existing newline handler rather than leaking
[57414;3u into the draft. Preserve installed twin mappings before consulting
pending aliases, matching first-writer-wins registration.
Replace the source PR's keyed branch ladder with a format table and verify
parser parity plus real buffer insertion with two invariant tests. Document
keypad multiline support in English and Chinese.
Live PTY: the exact doubled leak after a real collapsed paste reproduces on
main; all 21 editor cases pass with the fix, including ordinary Enter and
legacy Alt+Enter controls. Whitespace also reproduces on main: adjacent
characters are not the root cause.
Co-authored-by: Christos Despotakis <christos@despotak.is>
d8a07768c5