Unify the durable-meta IRI term classifier across ingest (durable-integrity) and responder read (graph-plan)

[Jurij89]

Context

#1921 (PR #1936) makes the "durable _meta subject must be an IRI" invariant span both the ingest and read paths, but the string-prefix term classifier is currently implemented in two private helpers:

• isIriMetaSubject in packages/agent/src/sync/durable-integrity.ts (ingest admission)
• isIriTerm in packages/agent/src/sync/responder/graph-plan.ts (responder read/paging)

They share the core contract (reject terms starting with _: or "), coupled today by a comment rather than by code.

Deliberate current difference (not a bug)

isIriMetaSubject('') returns false (fail-closed hardening: an empty/malformed _meta subject is dropped at admission), whereas the responder's isIriTerm('') returns true. This divergence is intentional and safe: the responder read path never sees an empty term post-ingest. It is called out here so unification is a conscious decision, not a silent behavior change.

Proposed follow-up

Extract a single canonical term-classifier in the layer that owns the string-encoded Quad contract (e.g. a shared sync/storage util) and import it from both durable-integrity and graph-plan. Decide explicitly how the shared helper treats the empty-string case (unifying to fail-closed would change the responder read path and needs its own review/tests). Cover the shared helper with IRI / blank-node / literal / empty-string cases.

Why deferred from #1921

#1921's scope is the ingest guard + its checkpoint-advance correctness. Unifying the classifier would reach into the responder read path (graph-plan.ts) and alter its empty-string handling — a separate, independently-reviewable change. Tracked here to keep #1921 focused.

Related: #1921, PR #1936. Raised by otReviewAgent on PR #1936 (durable-integrity.ts ~:1454 thread).

[Jurij89]

Fixed by #1944 (merged to testnet-canary as 7f78233).

Unified the two durable-meta IRI classifiers into one shared packages/agent/src/sync/iri-term.ts, imported by both durable-integrity (ingest) and responder graph-plan (read), with TWO named exports preserving the intentional, load-bearing divergence: isIriTerm (lenient, responder read; ''→true) and isIriMetaSubject (strict/fail-closed ingest; unknown param, ''/non-string→false). Behavior-preserving, byte-identical to the prior locals; new unit test asserts the divergence.