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/xlsx.py

@@ -0,0 +1,95 @@
+"""XLSX instrumenter — embed an external-image link.
+
+XLSX is structurally identical to DOCX (Office Open XML zip).  The
+injection target is the workbook's relationships file
+(``xl/_rels/workbook.xml.rels``).  We add an external image
+relationship there; Excel/LibreOffice fetch external images on
+workbook open in the same way Word does.
+
+We don't inject a ``<drawing>`` element into a sheet because that
+requires touching ``xl/worksheets/sheetN.xml`` *and* allocating a new
+``xl/drawings/drawingN.xml`` part — much higher chance of mangling
+the file.  An orphan external image relationship is enough: many
+Office viewers fetch all relationships at open time regardless of
+whether they're referenced from a sheet.
+
+If the operator wants a stronger trigger (image visible in the
+sheet, fetched even by viewers that lazy-load external resources)
+they should embed the slug as a hyperlink cell content via the
+``plain``/``passthrough`` instrumenters.
+"""
+from __future__ import annotations
+
+import io
+import zipfile
+from typing import Tuple
+
+from decnet.canary.base import (
+    CanaryArtifact,
+    CanaryContext,
+    CanaryInstrumenter,
+    InstrumenterRejectedError,
+)
+from decnet.canary.instrumenters.docx import _inject_relationship, _next_rid
+
+
+_RELS_PATHS = (
+    "xl/_rels/workbook.xml.rels",
+    "xl/_rels/sharedStrings.xml.rels",
+)
+
+
+def _mutate(blob: bytes, url: str) -> Tuple[bytes, str, str]:
+    try:
+        with zipfile.ZipFile(io.BytesIO(blob), "r") as zf:
+            members = [(zi, zf.read(zi.filename)) for zi in zf.infolist()]
+    except zipfile.BadZipFile as e:
+        raise InstrumenterRejectedError("uploaded blob is not a valid XLSX zip") from e
+
+    target_rels: str | None = None
+    for zi, _ in members:
+        if zi.filename in _RELS_PATHS:
+            target_rels = zi.filename
+            break
+    if not target_rels:
+        raise InstrumenterRejectedError(
+            "XLSX has no workbook relationships file to mutate"
+        )
+
+    out_members = []
+    rid = ""
+    for zi, data in members:
+        if zi.filename == target_rels:
+            rid = _next_rid(data)
+            data = _inject_relationship(data, rid, url)
+        out_members.append((zi, data))
+
+    out = io.BytesIO()
+    with zipfile.ZipFile(out, "w", zipfile.ZIP_DEFLATED) as zf_out:
+        for zi, data in out_members:
+            zf_out.writestr(zi, data)
+    return out.getvalue(), rid, target_rels
+
+
+class XlsxInstrumenter(CanaryInstrumenter):
+    name = "xlsx"
+    mime_prefixes = (
+        "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
+    )
+
+    def instrument(
+        self, blob: bytes, ctx: CanaryContext, *, target_path: str,
+    ) -> CanaryArtifact:
+        url = f"{ctx.http_base.rstrip('/')}/c/{ctx.callback_token}"
+        mutated, rid, target_rels = _mutate(blob, url)
+        return CanaryArtifact(
+            path=target_path,
+            content=mutated,
+            mode=0o644,
+            mtime_offset=-86400 * 14,
+            instrumenter=self.name,
+            notes=[
+                f"injected external-image relationship {rid} into "
+                f"{target_rels} -> {url}",
+            ],
+        )