fix(build): populate empty base artifacts; clarify README IRI wording (#463)

The -base release products shipped with zero METPO terms (verified empty in the
2026-05-19 release and pre-#462 main; full artifacts unaffected). Root cause: the
base pipeline used `remove --base-iri $(URIBASE)/METPO --axioms external`, but
METPO term IRIs are https://w3id.org/metpo/<id> (lowercase, slash), so the filter
never matched and every METPO axiom was stripped. The QC report rules already use
the correct $(URIBASE)/metpo. Changed line 542 to match; metpo-base.* now carries
all 1454 METPO classes (imported externals still excluded).

Also reworded the README IRI paragraph (Copilot review on #462) to distinguish the
resolver host (w3id.org) from the canonical identifier namespace
(https://w3id.org/metpo/).

Pre-existing defect, not introduced by #462. Fixes #463.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: turbomam

README.md

@@ -34,7 +34,7 @@ The canonical IRI for every METPO term is under the `https://w3id.org/metpo/` na
 - The ontology prefix `METPO:` expands to `https://w3id.org/metpo/`
 - Ontology document IRI: `https://w3id.org/metpo.owl` (release products and version IRIs live under `https://w3id.org/metpo/`, e.g. `https://w3id.org/metpo/releases/<date>/metpo.owl`)
 
-**Do not use `http://purl.obolibrary.org/obo/METPO_<id>`.** METPO is not registered in the OBO Foundry, so those PURLs do not resolve (HTTP 404) and are not METPO identifiers. `https://w3id.org/` is the only canonical resolution base for METPO. (Term-IRI resolution behavior is still being finalized; see issues #450 and #435.)
+**Do not use `http://purl.obolibrary.org/obo/METPO_<id>`.** METPO is not registered in the OBO Foundry, so those PURLs do not resolve (HTTP 404) and are not METPO identifiers. The canonical METPO identifier namespace is `https://w3id.org/metpo/` (resolved through the `w3id.org` host); `purl.obolibrary.org` is not used by METPO. (Term-IRI resolution behavior is still being finalized; see issues #450 and #435.)
 
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