Update ARC-20 for Leo 4.4.0

[mohammadfawaz]

Summary

• Set the ARC-20 library compiler version to Leo 4.4.0 and export IARC20 for dependent packages.
• Replace removed context accessors with std::ctx functions and add required constructors to the examples.
• Keep the specification interface synchronized with the library and ignore generated build/ output.

Verification

• Built IARC20 with leo 4.4.0 (320a595 HEAD).
• Confirmed that Leo generated the IARC20 interface ABI.
• Confirmed that the README interface block matches src/lib.leo.
• Compiled the dynamic-dispatch example with Leo 4.4.0.
• Ran git diff --check.

Formatter note

The source keeps export interface IARC20 on one line. Leo 4.4.0 currently formats all tested export placements incorrectly. ProvableHQ/leo#29618 tracks this formatter defect.

[vicsn]

Replace removed context accessors with std::ctx functions and add required constructors to the examples.

FWIW I don't see this part from the PR README