Unit 5: Add the blocking check mode

[mairas]

Target repo: halos-org/docs-tools

Goal: translation-status can fail a build. This is the unit that makes the
gate possible; it does not turn it on anywhere.

Requirements: R6, R7, R8, R9

Dependencies: Unit 2

Files:

• Modify: src/halos_docs_tools/translation_status.py
• Modify: tests/test_translation_status.py

Approach:

• Default behaviour is unchanged and still exits 0, so nothing that calls the
  command today changes meaning.
• The check mode exits non-zero when any entry is stale, missing,
  unstamped or orphaned, evaluated across the whole repository rather than a
  subset of pages.
• The failure output must name what failed. A non-zero exit with only a summary
  count forces the reader into the job log to learn anything.
• Consequence to document in the repo README, not to code around: adding a
  locale to mkdocs.yml makes every page missing in that locale until its
  pages land, so a new locale arrives in one pull request (R9).

Execution note: Test-first — each of the four failing states should have a
red test before the exit-status logic exists.

Test scenarios:

• Happy path: a fully current tree exits 0 in check mode.
• Error path: one stale page exits non-zero.
• Error path: one missing page exits non-zero.
• Error path: one unstamped page exits non-zero.
• Error path: one orphaned page exits non-zero.
• Edge case: a page that is current in eight locales and stale in the ninth
  still exits non-zero.
• Edge case: --only-pages does not narrow what the check mode considers —
  the gate stays repo-wide even when the report is filtered.
• Happy path: without the check flag, the same failing tree still exits 0.

Verification:

• Against a clone of halpi2 at main, check mode exits 0. Against PR
  #47's branch, it exits non-zero
  and names 108 entries.