[rntuple] Fix the circular import when rootfilespec.rntuple is imported first

[sathabbott]

🤖 AI generated content

Fixes #119.

import rootfilespec.rntuple.schema (or any rootfilespec.rntuple.* module) in a fresh interpreter failed: rntuple.envelope imports bootstrap.compression, which runs bootstrap/__init__.py, which imports RAnchor, which imported the half-initialized rntuple.envelope.

RAnchor only needs REnvelopeLocator, HeaderEnvelope, FooterEnvelope and LargeLocator inside header_locator, footer_locator, get_header and get_footer, so they are now imported there, and under TYPE_CHECKING for the annotations. No behaviour change.

Test: tests/test_imports.py imports every module of the package in its own interpreter (35 modules). On main, all the rntuple modules fail.

Found along the way, not fixed here: rootfilespec.container has the same kind of cycle (container → bootstrap/__init__ → bootstrap.array → container). It is marked xfail(strict=True) in the new test, so it will flag when fixed. Happy to fix it here or in a follow-up if you'd like; it isn't RNTuple, so I left it.

Full suite: 389 passed, 53 skipped, 47 xfailed.

Assisted-by: claude-code:claude-opus-5-5

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.83%. Comparing base (3f43287) to head (a64b38e).
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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #126   +/-   ##
=======================================
  Coverage   91.82%   91.83%           
=======================================
  Files          37       37           
  Lines        2962     2964    +2     
=======================================
+ Hits         2720     2722    +2     
  Misses        242      242           

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[nsmith-]

Found along the way, not fixed here: rootfilespec.container has the same kind of cycle (container → bootstrap/init → bootstrap.array → container). It is marked xfail(strict=True) in the new test, so it will flag when fixed. Happy to fix it here or in a follow-up if you'd like; it isn't RNTuple, so I left it.

Go ahead and fix the other case