docs: add FOUNDATIONS-BRIDGE.adoc (CC-BY-SA-4.0) under docs/ (#122)

The two-foundations integration brief — echo-types (residual-modality
axis, Agda Echo.*) + tropical-resource-typing (resource-grade axis, Lean
Resource.*) — as AsciiDoc under docs/ per the estate doc-format rule,
with a single-SPDX CC-BY-SA-4.0 prose header (ShareAlike, owner ruling
2026-06-04). Supersedes the root FOUNDATIONS_BRIDGE.md's non-conformant
multi-SPDX header.

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

Author: hyperpolymath