feat: mark writable-only scans in the result footer

[JeanExtreme002]

What

Replaces the two-line note that sat above every writable-only result set
with a short marker in the footer, under the rows it qualifies.

Before:

Note: Writable regions only — nothing in read-only memory was searched.
Use '--all-regions' on the first scan to include it.
+-----+--------------------+-------+
| ROW | ADDRESS            | VALUE |
+-----+--------------------+-------+
|  #1 | 0x00000201A4C0F118 | 95    |
+-----+--------------------+-------+
1 row in set (0.02 sec)

After:

+-----+--------------------+-------+
| ROW | ADDRESS            | VALUE |
+-----+--------------------+-------+
|  #1 | 0x00000201A4C0F118 | 95    |
+-----+--------------------+-------+
1 row in set — writable regions only (0.02 sec)

The caveat is about the rows, so it belongs next to the rows — that is where
the eye already is when it asks whether the result set is everything.

Where it shows up

Unchanged in reach: the scan itself, every refine, and every later
scan:results, whether the restriction came from --writable or from the
writable_only setting. An empty result set keeps it too —
Empty set — writable regions only — because "nothing found" and "nothing
found there" are different answers.

Implementation

Printer.table and Printer.footer gained an optional marker, appended to
the count line before the timing. Any listing can now carry a caveat about its
rows without a paragraph above them; scanning is the only caller so far.

Also in this PR

• The sample footers in help scan, help scanning and the docs showed a
  --writable scan without the marker, so they were updated.
• docs/guide/scanning.md and docs/troubleshooting.md described the old
  note and now describe the footer.
• The README/docs screenshot was regenerated: it shows the new footer, and it
  was still on the 0.1.0 banner.

Version

Bumped to 0.2.0. The footer under every result table changed shape, which
is visible to anyone reading it, so this is a minor rather than a patch.
picklock/__init__.py is the source hatch reads; docs/quickstart.md, the
Sphinx fallback in docs/conf.py and the terminal capture spell the version
out, so they move with it.

Checks

• pytest: 927 passed, including the end-to-end scans against a real process.
• make lint and make type-check clean.
• The end-to-end test now also asserts the marker appears after the table,
  not just that it appears.

[read-the-docs-community]

Documentation build overview

📚 Picklock | 🛠️ Build #34305386 | 📁 Comparing ab50823 against latest (49e5bcc)

  🔍 Preview build  

4 files changed

± quickstart.html
± troubleshooting.html
± guide/scanning.html
± reference/commands.html