Boxed table output for load-report

[Niccolum]

Motivation:
A text-format load report already exists (cli/format.py:format_text) and shows
field origins with masking. The only missing piece is a "boxed" table renderer
(+------+----------+-------+) that is easier to scan at a glance — purely cosmetic.

Proposal:
Add a format_table(report, ...) function in cli/format.py (or a --format=table
flag for dature inspect) that renders a compact ASCII table:

  +------------------+----------+--------------+
  | key              | source   | value        |
  +------------------+----------+--------------+
  | database.host    | app.yml  | db.prod.com  |
  | database.pass    | vault    | ****         |
  +------------------+----------+--------------+

Masking is already implemented; this is presentation only.

Acceptance criteria:

• Tabular output with key, source, value columns; secrets masked.
• Existing text and json formats unchanged.
• Tests asserting exact output (as per project convention — full ==, not in).
• changes/ fragment.

References: hoplite report mode.