Looking back honestly at the mistakes on this project:
Pascal fundamentals I should have caught immediately:
Seek record index vs byte offset (the 22 GB bug)
AnsiChar(x) needing Byte() intermediate — FPC is stricter but this is documented behaviour
Forward declarations can only be resolved in the same unit
TTimeSpec/TUnixTime/LocalTime — should have checked the FPC RTL units before writing a single line, not after compiler errors
CLOCK_MONOTONIC not exported reliably — same thing
Inline var declarations — I knew Delphi 7 doesn't support them and still introduced them repeatedly
case of string — same, knew it was invalid and still wrote it