fix(package): preserve pyenv roots and initialize Zsh explicitly

[ss-o]

Problem

Both pyenv package profiles overwrite PYENV_ROOT with the package checkout and generate initialization for that directory. Existing interpreter installations and user-selected roots are consequently ignored. Initialization also relies on shell inference.

Reproduced against package main 2243d87. Both profiles fail the existing-root assertion.

Approved correction

Preserve PYENV_ROOT, defaulting to HOME/.pyenv, and generate explicit Zsh initialization at load time using that root. Refresh command lookup after initialization. Use the native program profile by default without the unnecessary bgn requirement; retain the optional bgn profile. Update examples and add a focused regression.

This is a prerequisite for z-shell/z-a-meta-plugins#73. The dynamic initialization follows https://github.com/pyenv/pyenv#installation and avoids caching shell code for the wrong root. It is a correctness tradeoff, not an unmeasured startup-speed claim.

Validation

The prepared correction passes root preservation, project selection, shim resolution and shell override/unset against upstream eb36c1b1e8509861ca4689865d65cc2740404525 with a disposable interpreter fixture. Complete real Zi install, repeat and update checks before review. No Python interpreter installation is required.

Local changes, this labeled issue and Project 28 dependency setup are approved. Publication and issue closure are outside scope.

[ss-o]

Implemented locally: @py-utils keeps pyenv and selects its corrected first-party native profile. Both profiles preserve PYENV_ROOT, default to HOME/.pyenv, initialize Zsh explicitly at load time and refresh command lookup. The default profile no longer requires BGN; the optional BGN profile remains available.

Offline root/failure regressions fail on unchanged main and pass after the correction. Real native and BGN Zi profiles pass repeat loading, project interpreter shim resolution, pyenv shell override/unset, and update. Native install tested pyenv 2.8.5; BGN Git profile tested upstream eb36c1b1e8509861ca4689865d65cc2740404525. The BGN fixture requires its documented ZPFX/bin path to exist at Zi startup; an initially missing prefix was corrected before the successful rerun. No Python interpreter was compiled or downloaded; selection uses a disposable executable fixture.

Repository-configured Trunk checks pass. Documentation explains root migration, shim precedence and the deliberate runtime initialization cost; no speed claim is made. Native macOS/ARM64 and remote CI remain untested. The local package change is owned by #6 and still requires publication. No commit, push or issue closure was performed.