test: port the compiler e2e scenarios to spec fixtures

[ascandone]

Ports the scenarios of internal/compiler/e2e_test.go (#199) that the specs format can express to spec fixtures, under internal/interpreter/testdata/script-tests/e2e/. Spec fixtures run on every engine the corpus is wired to: the interpreter here, and the compiler+VM too once #199 lands, so each scenario is checked against both instead of the VM alone.

43 fixtures, covering: plain and inorder sources, caps, kept and nested destinations, source and destination allotments (including the leftover-rounding cases and an amount beyond int64), int and monetary arithmetic, get_amount / get_asset / balance() / overdraft(), account interpolation (string, number, and an account var whose origin is another account), bounded overdraft, send-all, save, colored sources, @world (literal and through a var), and oneof (flat and nested). Missing funds is asserted with expect.error.missingFunds.

Not ported, because the specs format can't express them: the cases expecting any other error (asset mismatch, invalid allotment sum, negative portion, negative balance, divide by zero, invalid color, unbounded @world in a send-all, a scoped account in an interpolation), and the two that count store balance reads. They stay as Go tests in #199.

All green on main. On #199, account-interpolation-account-alias is red: the VM posts to dest:world instead of dest:src. The fix will land in #199. This supersedes #208, which added only that one fixture.