feat(env): warn at runtime on deprecated forms

[Chemaclass]

🤔 Background

Related #866

Deprecations were recorded only in help text and docs, with no path out. The unprefixed
settings have been superseded since 0.15.0 (~23 months) and bashunit test --assert since
the assert subcommand landed — neither ever told a user at runtime, which is how a
deprecation stays in place indefinitely.

This adds the mechanism and the policy, without removing anything.

💡 Changes

• bashunit::env::warn_deprecated, wired to the unprefixed settings and --assert. Output goes to stderr so it cannot corrupt a report on stdout; BASHUNIT_NO_DEPRECATION_WARNINGS=true silences it
• Detection runs before the := block — afterwards the prefixed name is set either way and the two cases are indistinguishable — and is pure bash, since a compgen -v capture would cost a fork on the budgeted cold-start path
• The hand-maintained alias list is kept honest by an anti-drift test against env.sh, matching the approach already used for the shell completions
• Documents the policy: a deprecated form warns for at least one minor release before removal in a major one

⚠️ Worth knowing

-a/--assert is not a pure rename. It goes through cmd_test's parser, so it also
accepts --env and --no-parallel; bashunit assert forwards everything to the
assertion instead. I found this by migrating tests to the new form and watching them
fail. That gap has to close before the old form can actually be removed — recorded in the
code rather than left to be rediscovered.