fix(calibration): grade active content on URL shape, not construct type

v0.3.0 made the untrusted upload path unusable: measured on both doors, an
ordinary remote image fail_secure'd and an ordinary link/autolink/refdef
quarantined, so only documents without external references persisted.

Two independent defects compounded; neither fix works alone:

1. `markdown-image: HIGH` fired on any external image. The exfil primitive is a
   URL that moves bytes outward, not an image. `is_ordinary_url` now grades on
   shape - http(s)/protocol-relative, no query, no userinfo, no percent-escape,
   no opaque host label or path segment -> LOW; anything data-carrying keeps the
   carrier's severity. raw-html and data: URIs stay HIGH unconditionally.
   Opacity reuses entropy's primitives; floors calibrated against real doc URLs
   (worst legit token H=4.08, exfil segments 4.36-4.54) and frozen in
   calibration.

2. The quarantine_default floor fired on ANY finding, a premise that broke when
   every ordinary link became a finding. It now fires at MEDIUM+ - a no-op for
   every detector that shipped before 0.3.0 (no LOW/INFO exists), which is what
   makes this a patch rather than a minor.

The corpus blind spot that let this pass 522 green tests is closed: the FP
corpus carries realistic markdown and is asserted on the OUTPUT gate under
PRESET_USER_UPLOAD, with a counter-corpus of exfil-shaped URLs that must still
block. Beaconing and short opaque segments are conceded in LIMITATIONS and
asserted by the coverage matrix rather than papered over.

No new public API; no new preset (0.4.0 work); allow_reserved default unchanged.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "llm-ingestion-guard"
-version = "0.3.0"
+version = "0.3.1"
 description = "A minimal, dependency-light defensive layer for LLM ingestion pipelines — the write-time siblings of query-time chatbot guardrails."
 readme = "README.md"
 requires-python = ">=3.10"