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.

decnet/canary/instrumenters/html.py

@@ -0,0 +1,45 @@
+"""HTML instrumenter — append a 1×1 tracking pixel.
+
+Stdlib-only.  We don't parse the HTML; we just inject the ``<img>``
+tag immediately before the closing ``</body>`` (or, failing that, at
+the end of the document).  Most renderers that support remote images
+(email previewers, IDE doc previews, browsers) will fetch it as
+soon as the document is opened.
+"""
+from __future__ import annotations
+
+import re
+
+from decnet.canary.base import CanaryArtifact, CanaryContext, CanaryInstrumenter
+
+
+_BODY_CLOSE = re.compile(rb"</body\s*>", re.IGNORECASE)
+
+
+class HtmlInstrumenter(CanaryInstrumenter):
+    name = "html"
+    mime_prefixes = ("text/html", "application/xhtml+xml")
+
+    def instrument(
+        self, blob: bytes, ctx: CanaryContext, *, target_path: str,
+    ) -> CanaryArtifact:
+        url = f"{ctx.http_base.rstrip('/')}/c/{ctx.callback_token}".encode()
+        pixel = (
+            b"<img src=\"" + url + b"\" width=\"1\" height=\"1\" "
+            b"alt=\"\" style=\"display:none\">\n"
+        )
+        match = _BODY_CLOSE.search(blob)
+        if match:
+            out = blob[:match.start()] + pixel + blob[match.start():]
+            note = "injected 1x1 pixel before </body>"
+        else:
+            out = (blob if blob.endswith(b"\n") else blob + b"\n") + pixel
+            note = "appended 1x1 pixel (no </body> found)"
+        return CanaryArtifact(
+            path=target_path,
+            content=out,
+            mode=0o644,
+            mtime_offset=-86400 * 7,
+            instrumenter=self.name,
+            notes=[note, f"pixel src={url.decode()}"],
+        )