feat(canary): operator-upload instrumenters + tests

Seven instrumenters that mutate operator-supplied artifacts to
embed the callback URL:

- passthrough — bytes unchanged; only DNS-callback tokens trip
  detection, with the slug embedded in the placement path
- plain      — substitutes {{CANARY_URL}}/{{CANARY_HOST}} placeholders;
  falls back to appending a comment line whose prefix adapts to the
  apparent file syntax (#, //, ;)
- html       — injects a 1x1 tracking pixel before </body>, appends
  if the close tag is missing
- docx       — direct zipfile manipulation (no python-docx dep):
  inserts an external-image Relationship into word/_rels/document.xml.rels
  and a matching <w:drawing> element before </w:body>
- xlsx       — sibling of docx; injects an external-image relationship
  into xl/_rels/workbook.xml.rels (orphan rels are still fetched on
  open by most viewers)
- pdf        — uses pikepdf to install /OpenAction /URI on the catalog;
  rejects with a clear message when pikepdf isn't installed
- image      — uses Pillow to embed slug + URL in PNG tEXt / JPEG
  comment; rejects with a clear message when Pillow isn't installed

DOCX and XLSX share the rId allocator + relationship injector via
the docx module; both work on stdlib zipfile only.

Tests synthesise minimal real DOCX/XLSX fixtures inline, round-trip
each instrumenter, and assert the callback URL ends up in the
mutated bytes while the file still parses.

tests/canary/conftest.py

@@ -0,0 +1,88 @@
+"""Shared fixtures for canary tests — minimal DOCX/XLSX/HTML/PDF fixtures.
+
+We synthesise the OOXML zips inline rather than checking real binary
+fixtures into the repo.  Keeps the test surface portable and the diff
+reviewable; the smallest valid DOCX is ~12 files but Word/LibreOffice
+both accept a stripped-down skeleton with just ``[Content_Types].xml``,
+``_rels/.rels``, ``word/document.xml``, and ``word/_rels/document.xml.rels``.
+"""
+from __future__ import annotations
+
+import io
+import zipfile
+
+import pytest
+
+
+_DOCX_CONTENT_TYPES = (
+    '<?xml version="1.0" encoding="UTF-8" standalone="yes"?>'
+    '<Types xmlns="http://schemas.openxmlformats.org/package/2006/content-types">'
+    '<Default Extension="xml" ContentType="application/xml"/>'
+    '<Default Extension="rels" ContentType="application/vnd.openxmlformats-package.relationships+xml"/>'
+    '<Override PartName="/word/document.xml" '
+    'ContentType="application/vnd.openxmlformats-officedocument.wordprocessingml.document.main+xml"/>'
+    '</Types>'
+)
+
+_DOCX_PACKAGE_RELS = (
+    '<?xml version="1.0" encoding="UTF-8" standalone="yes"?>'
+    '<Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships">'
+    '<Relationship Id="rId1" '
+    'Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/officeDocument" '
+    'Target="word/document.xml"/>'
+    '</Relationships>'
+)
+
+_DOCX_DOCUMENT = (
+    '<?xml version="1.0" encoding="UTF-8" standalone="yes"?>'
+    '<w:document xmlns:w="http://schemas.openxmlformats.org/wordprocessingml/2006/main">'
+    '<w:body><w:p><w:r><w:t>Existing content.</w:t></w:r></w:p></w:body>'
+    '</w:document>'
+)
+
+_DOCX_DOCUMENT_RELS = (
+    '<?xml version="1.0" encoding="UTF-8" standalone="yes"?>'
+    '<Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships">'
+    '</Relationships>'
+)
+
+
+@pytest.fixture
+def minimal_docx() -> bytes:
+    """Return a tiny but structurally valid DOCX as bytes."""
+    out = io.BytesIO()
+    with zipfile.ZipFile(out, "w", zipfile.ZIP_DEFLATED) as zf:
+        zf.writestr("[Content_Types].xml", _DOCX_CONTENT_TYPES)
+        zf.writestr("_rels/.rels", _DOCX_PACKAGE_RELS)
+        zf.writestr("word/document.xml", _DOCX_DOCUMENT)
+        zf.writestr("word/_rels/document.xml.rels", _DOCX_DOCUMENT_RELS)
+    return out.getvalue()
+
+
+_XLSX_CONTENT_TYPES = (
+    '<?xml version="1.0" encoding="UTF-8" standalone="yes"?>'
+    '<Types xmlns="http://schemas.openxmlformats.org/package/2006/content-types">'
+    '<Default Extension="xml" ContentType="application/xml"/>'
+    '<Default Extension="rels" ContentType="application/vnd.openxmlformats-package.relationships+xml"/>'
+    '<Override PartName="/xl/workbook.xml" '
+    'ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet.main+xml"/>'
+    '</Types>'
+)
+
+_XLSX_WORKBOOK_RELS = (
+    '<?xml version="1.0" encoding="UTF-8" standalone="yes"?>'
+    '<Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships">'
+    '</Relationships>'
+)
+
+
+@pytest.fixture
+def minimal_xlsx() -> bytes:
+    """Return a tiny but structurally valid XLSX as bytes."""
+    out = io.BytesIO()
+    with zipfile.ZipFile(out, "w", zipfile.ZIP_DEFLATED) as zf:
+        zf.writestr("[Content_Types].xml", _XLSX_CONTENT_TYPES)
+        zf.writestr("_rels/.rels", _DOCX_PACKAGE_RELS.replace("word/document.xml", "xl/workbook.xml"))
+        zf.writestr("xl/workbook.xml", '<workbook/>')
+        zf.writestr("xl/_rels/workbook.xml.rels", _XLSX_WORKBOOK_RELS)
+    return out.getvalue()