docs: add SUSTAINABILITY.md long-term backlog

Captures a full-repo sustainability review as a tracked backlog:
ABI / SDK / datastore / wire-format / build items with stable IDs,
severity, reversibility, version impact, verified file:line evidence,
and a suggested sequencing. Tier 0 flags the irreversible-once-adopted
items (enum representation across the C ABI, a wire/store format-version
anchor, size_t in ABI structs); the existing mechanical ABI discipline
is recorded as context to preserve.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: facontidavide