Preserve sticky module requirements during purge

[adrien-cotte]

Purge skips sticky and super-sticky modules but still attempts to unload their non-sticky requirements. This reports an unrelated dependency error without explaining why the requirement remains loaded.

Preserve requirements needed by retained sticky modules and apply the sticky_purge reporting mode to their skipped unload.

Fixes #582

Assisted-by: Codex:gpt-6-astra

[xdelaruelle]

Testsuite should be fixed as CI jobs are failing.

Documentation should be adapted to mention this behavior change, especially in module.rst, changes.rst, the sticky/module-tags design docs and the sticky-modules cookbook.

Also good to check if this has an impact on other stuff mentioning sticky modules like stash-environment, initial-environment, conflict_unload, evaluation-errors. Check these design doc and adapt them if required.

[xdelaruelle]

While reviewing I found two gaps in the new protection, so I rebased the branch on current main and pushed three follow-up commits on top of yours:

• Keep sticky requirement of super-sticky module on forced purge. A requirement tagged sticky and needed by a retained super-sticky module got two contradictory messages on purge -f: "Unload of sticky module forced" immediately followed by "Unload of super-sticky module requirement skipped". The sticky check ran first and reported the forced unload before the requirement check retained the module. The requirement check now lives inside failOrSkipUnloadIfSticky, ahead of the forced-unload warning. It also reuses isModuleSticky for the dependent stickiness rule and drops the extra call in cmdModuleUnload.
• Protect sticky module requirement whatever its load order on purge. The dependent lookup used the positional-order dependency hash, so a requirement loaded after its super-sticky dependent (for instance after module unload -f req then module load req) was purged and left the dependent with a missing prereq, which is the very situation #582 describes. The lookup now uses the non-positional-order hash. Tests cover both load orders, and the interplay with abort_on_error when the skipped requirement raises an error. A bullet in the sticky-modules design doc records this.
• doc: fix NEWS placement and cookbook example. The NEWS entry moves to the end of the 5.7.0 list, as entries follow the order changes landed. The cookbook example now shows the output of module load env/base, whose requirements are auto-loaded, and the tags they carry in module list.

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