refactor: move MAX_MACH to aircraft design namespace

[suhaslord]

Implements the repository-wide foundation for OpenMDAO#1046.

Why

MAX_MACH is an aircraft design property, but Aviary currently exposes and serializes it as Mission.Constraints.MAX_MACH. That leaks a mission-scoped name into FLOPS pre-mission sizing, aircraft input decks, validation data, and tests.

What changed

• introduces canonical Aircraft.Design.MAX_MACH = 'aircraft:design:max_mach'
• removes the duplicate mission-scoped Python hierarchy symbol so Aviary's variable namespace remains unique
• migrates all in-repo Python consumers to Aircraft.Design.MAX_MACH
• migrates aircraft CSVs, converter fixtures, validation models, JSON, and documentation notebook data to aircraft:design:max_mach
• preserves compatibility for existing serialized input decks by normalizing the legacy raw key mission:constraints:max_mach before AviaryValues storage/access and CSV metadata validation
• moves the metadata record into the alphabetically correct Aircraft.Design position and documents its FLOPS consumers
• adds a migration contract test that prevents unapproved legacy references from reappearing
• adds an idempotent migration/audit tool and a machine-readable manifest

Migration audit

The generated manifest records 50 migrated repository files, 57 Python-symbol replacements, and 24 serialized-key replacements. Compatibility-only files are deliberately excluded from the no-legacy-reference ratchet.

Scope

This is a namespace/data-model migration. The FLOPS equations themselves are unchanged. Full Aviary CI is validating numerical behavior and legacy input-deck compatibility before any upstream submission.

[cursor]

Fixed the "Verify MAX_MACH migration idempotence" CI check failure.

Issues Fixed

1. patch_metadata() non-idempotence: The function was removing and re-inserting the Aircraft.Design.MAX_MACH metadata block on every run, which added extra blank lines before Aircraft.Design.MAX_STRUCTURAL_SPEED. The fix adds an early-return check that detects when MAX_MACH is already correctly positioned with the expected description.

2. Manifest overwrite on no-op runs: The historical 50-file manifest was being replaced with a trivial 1-file record on idempotent re-runs. The fix now only rewrites the manifest when actual symbol/key replacements occur (python_symbol > 0 or serialized_key > 0).

Verification

The migration tool now passes the CI check:

python3 tools/migrate_max_mach_namespace.py --write --check
git diff --exit-code  # Clean on second run

Both runs produce zero git diff on codebase files, ensuring true idempotence.

[cursor]

✅ CI checks now passing

Both required checks are now green:

• ✅ Verify MAX_MACH migration idempotence (run #32555923081)
• ✅ pre_commit / ruff-format (passed in Aviary Tests)

The formatting fix ensures the migration script follows the ruff code style while maintaining idempotence.