fix(unload): reach the Plugin Standard unload function

[ss-o]

Makes the Plugin Standard unload function reachable. Fixes the reported case and a second one found while building z-shell/zd#119.

The defect

(( ${+functions[${plugin}_plugin_unload]} )) && ${plugin}_plugin_unload

$plugin was used verbatim, so two kinds of plug-in could never be reached.

Hyphenated repositories, the reported case. ADR-0020's namespace rules require the shell prefix to use underscores, so zsh-eza can define zsh_eza_plugin_unload and satisfy the standard, or define zsh-eza_plugin_unload and be unloadable by Zi. Not both.

Path-loaded plug-ins, which were not in the report. Such a plug-in is identified as %<absolute path> and $plugin is the directory, so the derived name is /path/to/thing_plugin_unload, which names nothing. Demonstrated while adding unload coverage:

PLUGINS_LIST: %/tmp/.../ulprobe
UNLOADED: func=0 alias=0 fpath_restored=1     # ulprobe_plugin_unload never ran
Running plugin's provided unload code: print -r -- STD-HOOK-RAN   # this did

@zsh-plugin-run-on-unload worked in both cases, which is why the gap stayed invisible: plug-ins using the callback were fine, and only the named-hook contract was broken.

The fix

Try candidate names in order and call the first that exists:

1. the literal name, so plug-ins already defining it keep working
2. the underscore form, which is the only name ADR-0020 permits a hyphenated plug-in to define
3. and 4. the same pair derived from the directory basename, the closest analogue of a repository name for a path-loaded plug-in

Duplicates collapse through (u), and exactly one function is called, since the Standard defines a single unload function per plug-in.

The change is additive. No name that resolved before stops resolving.

Test

tests/unload-hook-dispatch.zsh loads and unloads two plug-ins, one plainly named and one hyphenated, each defining only the name its own rules permit, then asserts both hooks ran. Against the previous dispatch:

the unload function of a path-loaded plug-in was not called: []
not ok - the Plugin Standard unload function was not called      exit=1

Registered in zsh-n.yml, both path filters and a job. Full suite 21/21.

Note on scope

This restores the hook's reachability. It does not change what the hook is expected to do, and it does not touch #113 or #483, which are separate unload defects. z-shell/zd#119 now asserts the four unload contracts that already worked, so a case for each of those can land with its fix.

Closes #482