refactor(core): remove the duplicate helper and correct drifted headers

[ss-o]

Wave 1 item 2 from the preserved architecture audit, plus the structural tests that stop both defects recurring.

The duplicate

.zi-at-eval was defined in both lib/zsh/autoload.zsh:303 and lib/zsh/install.zsh:2047. I diffed them: same behaviour, different wording. So nothing was broken.

What makes it worth fixing is that both libraries are sourced conditionally and in different orders depending on the command:

zi.zsh:1383  (( ${+functions[.zi-setup-plugin-dir]} )) || source .../install.zsh
zi.zsh:1384  (( ${+functions[.zi-confirm]} ))          || source .../autoload.zsh
zi.zsh:1393  (( ${+functions[.zi-setup-plugin-dir]} )) || source .../install.zsh   # install only

So which definition wins varies by code path. Two copies that must stay in lockstep, selected non-deterministically, become a heisenbug the first time they drift.

Removing the autoload.zsh copy is safe: the only call sites are install.zsh:2410 and install.zsh:2430, inside the file that defines it; it is absent from contracts/public-contract-v1.json; and it has no consumer anywhere in the organization.

The drifted headers

lib/zsh/autoload.zsh   header '.zi-update-in-parallel'  precedes  '.zi-update-all-parallel'
zi.zsh                 header '+zi-parse-opts'          precedes  '.zi-parse-opts'

The second is not in the issue. A systematic sweep of every # FUNCTION: header against the definition that follows it turned it up, which is why the test does that sweep rather than checking one known case.

Why a structural test

Neither defect is reachable behaviourally. Duplicate definitions that agree behave correctly right up until they drift, and a wrong header misleads only the next reader. tests/source-hygiene.zsh asserts both directly over zi.zsh and lib/zsh/*.zsh, currently 175 functions.

Each defect was reintroduced separately and the test observed failing on it:

functions defined more than once:
  .zi-at-eval: lib/zsh/autoload.zsh:304 and lib/zsh/install.zsh:2047
not ok - a function is defined in more than one shipped source        exit=1

FUNCTION headers naming a different function:
  lib/zsh/autoload.zsh:1912 header '.zi-update-in-parallel' precedes '.zi-update-all-parallel'
not ok - a FUNCTION header does not match the function it precedes    exit=1

Registered in zsh-n.yml, both path filters and a job.

Two issue items not addressed, deliberately

Item 2 is not reproducible. The header above @zsh-plugin-run-on-unload at zi.zsh:1309 names that function correctly. Either it was fixed after the audit or the audit was slightly off. Nothing to change, and the new test would catch it if it regressed.

Item 4 is out of scope. The .zi-load-ices snippet-directory TODO is untouched; the issue itself says it needs an ID-collision test before it can be classified as a defect. That deserves its own investigation rather than being bundled here.

Full suite 20/20, zsh -n clean on zi.zsh and both libraries.

Closes #449