Align the engine pin with upstream geistlib and drop the compat patch

[geisten]

Problem

geist-memory pins geistlib 9030b783 and applies a mandatory local patch. Hosts that already link geistlib for generation at another revision cannot add geist-memory without linking two engines or carrying the patch themselves.

Request

• Once Upstream (or retire) geist-memory's compat patch so consumers can share one unpatched engine pin geistlib#521 lands (compat fixes upstreamed or confirmed obsolete), move GEIST_REV to a main revision without patches/geistlib-compat.patch.
• Allow building the adapter against a caller-provided geistlib tree/archive (e.g. GEIST_ROOT= / GEIST_LIBRARY=) so a host can use its single pinned engine; keep make deps for standalone use.
• CI: build and run the E2E test against the pinned revision and against "latest main" (allowed to fail, reported) to detect drift early.

Acceptance

• make deps no longer applies an engine patch.
• Adapter + host link with exactly one libgeist.
• E2E embedding test passes on the new pin; the documented model benchmark numbers are re-measured or marked stale.

Depends on geisten/geistlib#521. Downstream: git.geisten.net/geisten/helio#7.