feat: flip swap mechanism on sagitta — MD as primary, RST as override

Mirror of #1899 (current) and #1900 (circinus) for sagitta. Same logic,
same scripts, per-branch file set.

Changes:
- Rename docs/**/md-<stem>.md to docs/**/<stem>.md (drop md- prefix) for
  all 210 stems previously listed in docs/_swap.txt
- Rename docs/**/<stem>.rst to docs/**/rst-<stem>.rst (add rst- prefix)
  for the same 210 stems
- Repurpose docs/_swap.txt as docs/_rst_overrides.txt; initially empty
- conf.py exclude_patterns flipped: rst-*.rst excluded by default
- conf.py runtime-artifact references updated to _rst_override_state.json
  and _md_exclude.txt
- scripts/swap_sources.py imported from current (post-#1899 rewrite, with
  inverted rename direction)
- scripts/import_myst.py and tests/test_import_myst.py deleted (obsolete)
- tests/test_swap_sources.py imported from current (post-#1899 rewrite)

Side-effect: fixes the same 404 on /en/1.4/ View page source links that
#1899 fixed for /en/rolling/ and #1900 fixed for /en/1.5/.

Per-branch differences vs #1899:
- sagitta has 210 stems vs current's 254 (sagitta has no vpp pages and
  fewer current-only features; cli + installation/cloud/aws are still
  RST-only on sagitta pending the title-level fix follow-up)
- otherwise the script/conf.py/test changes are byte-identical with current

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