chore(license): add a third header variant for OpenCV-derived sources

Summary

scripts/check-license-headers.mjs currently knows two header variants:
derived (LGPL + the upstream jsfeat MIT attribution) and original (LGPL
only). Code ported from OpenCV fits neither. Add a third variant before the
first such file lands.

Why

The script treats ORIGINAL_SRC as a short allowlist and documents the rest as:

Everything else under src/ is a port of the corresponding jsfeat code.

That was true until now. It stops being true with the incoming features2d
work: TEBLID, GMS and FREAK are ported from OpenCV's xfeatures2d, not from
jsfeat. Under the current rules a new src/teblid/teblid.ts would automatically
receive the MIT attribution to Eugene Zatepyakin.

That is a false credit — precisely the failure the two-variant policy exists
to prevent, in the opposite direction. AGENTS.md already spells out the
principle:

adding the jsfeat attribution here would be a false credit, and licence
scanners would report MIT content in files that have none.

The same reasoning applies symmetrically: OpenCV-derived files must carry the
OpenCV notice and must not carry jsfeat's.

Proposed change

Add a third case producing the LGPL base without the jsfeat attribution,
plus the upstream OpenCV notice — the same shape as the hand-extended headers
already tracked in EXTENDED for src/orb/bit_pattern_31.ts (OpenCV/ORB) and
src/math/math.ts (*BSD qsort).

Mechanically this can be either a new template variant or an explicit path set
checked like EXTENDED; the implementer should pick whichever keeps the licence
wording composed from the single template in
.agents/skills/license-header-adder/resources/, so the text cannot drift.

Verify the upstream licence per file — do not assume

opencv_contrib/modules/xfeatures2d mixes 3-clause BSD and Apache-2.0 file
headers depending on the file and its contribution date. Both are one-way
compatible with LGPL-3.0-or-later, so relicensing is fine, but the notice text
differs and must match the file actually ported. Check the header of each
upstream source before writing the notice.

Acceptance criteria

• A new file under src/teblid/ (or any path designated OpenCV-derived)
  passes node scripts/check-license-headers.mjs.
• The applied header contains no reference to jsfeat or Eugene
  Zatepyakin, and does contain the upstream OpenCV notice.
• --fix applies the correct variant automatically, chosen by path.
• The existing hand-extended notices in src/math/math.ts and
  src/orb/bit_pattern_31.ts are untouched and still pass.
• The policy is documented in
  .agents/skills/license-header-adder/SKILL.md, including the "verify the
  upstream licence per file" rule.

Out of scope

• Any algorithm implementation.
• Revisiting the licence of existing files.
• tests/vendor/** and examples/js/ third-party scripts, which are never
  stamped.

Related

• Header policy: #112
• First consumers: #135 (TEBLID), #134 (GMS) and #80 (FREAK)
• Plan: docs/features2d-expansion-plan.md

Start with scripts/check-license-headers.mjs, the single template in .agents/skills/license-header-adder/resources/, and .agents/skills/license-header-adder/SKILL.md. Inspect the upstream headers for each OpenCV-derived source before defining path-based selection, then run node scripts/check-license-headers.mjs and --fix against the listed existing and new paths. Done means the OpenCV notice is applied without jsfeat attribution, documentation is updated, and existing extended notices still pass.