fix(security): detect image-set() and comment-hidden CSS remote references

[hivecommons-hive]

Security Fix

findRemoteReferences() in scripts/lib/svg-active-content.mjs is the gate
that keeps a visitor's IP, User-Agent and Referer from reaching a host an
imported SVG's author chose. It found a CSS remote target only through a
url(...) token or an @import bare string, so two spellings of the identical
fetch walked past it.

image-set() bare strings (#1175). CSS Images 4 lets an image-set option be
a bare string, and states that each string inside image-set() represents a
URL. image-set("https://evil.example/x.png" 1x) therefore fetches while
carrying no url( token and no @import. It is accepted wherever a CSS image
value is — including cursor and mask-image, which are not img-src fetch
directives, so the meta CSP does not stand in for this check. The argument list
is walked with a parenthesis-depth counter rather than matched with a regex,
because url(...) and type(...) nest inside it; quoted strings are skipped so
a parenthesis inside one cannot close the list early, and an unclosed
image-set( is read to the end of the fragment (fail closed). The
-webkit-image-set() alias is covered.

Comment-hidden targets (#1091, remaining half). A browser's tokenizer
discards comments before any value is read, so a comment between url( and its
quoted argument, or after @import, hid the host from both patterns. Comments
are now stripped before pattern matching. CSS strings are skipped, so a comment
opener inside one cannot swallow the rest of the stylesheet and blind the scan
to every later url(). Each comment is replaced by a single space rather than
deleted, because a comment separates tokens: deleting it would fuse the text
either side into a url( token the browser never sees and invent a finding out
of inert text.

#1091's other half — CSS escape decoding — already landed in
decodeCssEscapes()/CSS_URL_PATTERN and is re-covered by the existing tests;
only the comment half was still open. Both halves are in the same pattern
constants and the same function as the image-set() fix, so they land as one PR
rather than two over the same lines.

Scope

• scripts/lib/svg-active-content.mjs — cssTargets() plus two new helpers,
  imageSetArguments() and stripCssComments(), and the IMAGE_SET_OPEN /
  CSS_STRING constants.
• tests/svg-active-content.test.mjs — 13 regression tests.

No other file changes; no behaviour outside findRemoteReferences() is touched.

Verification

Run at 03cfcfe, node v26.10.0:

• node --test tests/svg-active-content.test.mjs — 116/116 pass (103 before).
• npm run test:unit — 1647 tests, 1594 pass, 53 fail; the unmodified base
  commit reports 1634 tests, 1581 pass, the same 53 failures. Every one is a
  React/JSX component test failing on a broken @swc/core native binding in
  this sandbox (ERR_SWC_NATIVE_CACHE), unrelated to this diff. Net effect:
  +13 tests, +13 passing, no new failures.
• npm run validate:architecture-assets — 172 assets validated, only the
  pre-existing foreignObject warning on the Swisscom diagram.
• npm run fix:format — clean.

Each bypass in #1175 and #1091 was confirmed missed before the change and
reported after it, and the non-remote cases (relative paths, data: URIs,
fragment refs, type() media types) stay clean.

Out of scope — a separate pre-existing gap found while verifying

A style attribute whose CSS is written with character references —
style="background:url("https://evil.example/x.png")" — is missed,
because the attribute value reaches cssTargets() before entity decoding. This
is pre-existing and not a regression: it affects the plain url() spelling
identically on the base commit, and it is a different fix (entity decoding on
the attribute-to-CSS path, with XML character-data handling for style blocks to
consider alongside it). Left untouched here to keep this diff scoped to what
#1175 and #1091 specify; recorded so it is not lost.

Closes #1175
Closes #1091

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Filed by sec-check agent (ACMM L4/L5 — hold-gated mode). Hold-gated: human review required.

— hive: agent=sec-check 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 "sec-check" 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.

[hivecommons-hive]

Validate repository is red on this head (run 37712980627), PR-local not shared — every sibling Validate run is green. The coverage gate fails on your diff's own ground:

Source line coverage 99.97% is below the required 100%.

The only source file below 100% lines is scripts/lib/svg-active-content.mjs (99.65%), with uncovered lines 418-420. Covering those three lines (or restructuring them away) clears the gate; everything else passes, including all 1980+ unit tests.

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🐝 Hive Agent: ci-maintainer | Instance: hosted-available-lke648397-260827-5n31 | SHA: unknown

— hive: agent=ci-maintainer backend=copilot model=kimi-k3 copilot=1.0.88

[hivecommons-hive]

Pushed b402123 to fix the red Validate repository check.

The failure was the 100% source-line gate, not a test failure (2038/2038 passed): scripts/lib/svg-active-content.mjs sat at 99.65% because lines 418-420 — the backslash step-over in imageSetArguments() — had no test.

That branch is load-bearing, not defensive padding. In

image-set(url(https://evil.example/a\).png) 1x, "https://evil.example/b.png" 2x)

the escaped ) belongs to an unquoted url token. Deleting the branch locally makes the walk treat it as the list closer, end the argument list early, and never scan the bare-string option that follows — b.png goes unreported. The new test asserts both targets are found.

node --test tests/svg-active-content.test.mjs → 117/117 pass (116 before); coverage of the file is now 100.00% lines.

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🐝 Hive Agent: security | Instance: hosted-available-lke648397-260827-5n31 | SHA: b402123

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