test: pin the uri-safety classifier's own contract

[hivecommons-hive]

Test Improvement

Adds tests/uri-safety.test.mjs — the first test that imports
scripts/lib/uri-safety.mjs directly.

That module is the shared decision point for both content gates
(scripts/lib/svg-active-content.mjs:24 and
scripts/lib/mdx-active-content.mjs:29 both import from it), and its header
states that a divergence between the two gates is "a bypass of whichever gate is
more permissive". Until now no file under tests/ imported it: every assertion
about it was made through findActiveContent / findRemoteReferences on a whole
SVG or Markdown document, so the predicate's own boundaries were reached only
through whichever values those fixtures happened to carry.

20 tests, pinning today's behaviour of four exports:

• decodeEntities — hexadecimal and decimal references with and without the
  terminating semicolon; the out-of-range guard (� and �
  are handed back unchanged rather than thrown out of String.fromCodePoint);
  all seven named references; an unrecognised name left as written.
• normalizeUri — the deliberate second decode pass (: → :),
  whitespace and control-character stripping, lowercasing, and non-string
  coercion.
• activeScheme — each of the four script-executing schemes; reference- and
  control-character-obscured forms; the scheme-boundary rule in both
  directions (url(javascript:…) matches, xjavascript:, not-javascript:,
  x.javascript: and x+javascript: do not); the same boundary for data:;
  all nine active data: media types plus the +xml suffix rule; the inert
  types (image/png, font/woff2, data:,) left alone.
  Also records that foo javascript:alert(1) is not flagged — normalizeUri
  removes the space before the boundary test, so no scheme boundary survives.
  That answer was previously unstated anywhere.
• describeTarget — both sides of the 120-character cut, including that the
  truncated form is itself exactly 120 characters.

No production code changes; this records existing behaviour only.

Scope and open-PR overlap

remoteTarget is deliberately excluded. #1249 is open and changes exactly
that function (slashless http: authorities) along with
tests/svg-active-content.test.mjs. The four exports covered here are untouched
by #1249 and by #1255, and this PR adds one new file and edits nothing else, so
the diffs do not overlap. A follow-up can extend this file to remoteTarget
once #1249 lands.

Verification

Repository revision 365e11f (main), node v26.10.0, TZ=UTC:

• node --test tests/uri-safety.test.mjs → 20 pass, 0 fail.
• npm run test:unit:coverage:check → exit 0;
  src files | 100.00 | 100.00 | 9015/9015 lines | 2715/2715 regions
  (region denominator rises from 2713 as the direct import attributes
  uri-safety.mjs's regions to the file itself), harness files and
  all files unchanged.
• npx prettier --check tests/uri-safety.test.mjs → clean.

Related Issue

Closes #1261

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Filed by quality agent (hold-gated mode). Human review required.

— hive: agent=quality backend=copilot model=claude-opus-5 copilot=1.0.88

[hivecommons-hive]

Important

Held for human review by the hive's ACMM level gate.

This PR was opened by the "quality" agent while Hive policy required a human checkpoint for that agent. Non-outreach agents are held at ACMM L3–L5; the outreach agent is always held because it publishes project-facing communication.

Hive will keep the hold label until a human removes it. Operators can make a deliberate one-off release during an ACMM level change with release_level_holds=true, but level changes never release this hold automatically.