feat(annex): add @zi-unregister-annex

[ss-o]

Adds @zi-unregister-annex, the missing counterpart to @zi-register-annex.

The defect

An annex following the Zsh Plugin Standard unload contract removes its handler. With no unregister call, its ZI_EXTS entry survives and the before-load dispatch calls a name that no longer exists.

That is not inert. A missing function returns 127, 127 & 1 is true, so Zi folds the error into its return value and shifts its argument list. Reproduced by loading a plug-in after an annex removed its handler:

zi:176: command not found: probe_annex_handler
Warning: Bad new-ices returned from the annex: probe-annex
Error: No plugin or snippet ID given.
RC=2

The plug-in does not load at all: the shift consumed its id. Every later load in the session is affected. That is why z-shell/z-a-meta-plugins currently leaves an inert stub behind instead of removing its handler.

The API

@zi-unregister-annex <annex-name> <hook-type>

Removes the matching ZI_EXTS and ZI_EXTS2 entries. It matches on the recorded name and type rather than reconstructing the key, because a hook: registration appends a sequence number to its key. Unregistering something never registered is a no-op, not an error.

One thing it deliberately does not do

The ice-modifier lists in ZI_EXTS[ice-mods] and ZI_EXTS2[ice-mods] are not unwound. Registration appends to a joined string without recording which annex contributed which entry, so removing one annex's share would require that association to be recorded at registration time first.

A leftover ice name is recognised but dispatches to nothing, which is harmless. The dispatch entry removed here is the part that corrupts loads. This is stated in the function comment rather than left for someone to discover.

Test

tests/annex-unregister.zsh registers a hook, unregisters it, removes the handler, then loads two plug-ins and asserts both statuses, both bodies, and that no ZI_EXTS entry survives. The second load is what proves the argument list was not shifted by the first.

With the unregister made inert:

loading after unregister returned 2: zi:176: command not found: probe_annex_handler
Error: No plugin or snippet ID given.
not ok - an unregistered annex hook still affects later plug-in loads     exit=1

Registered in zsh-n.yml, both path filters and a job. Full suite 22/22.

Follow-up not in this PR

ecosystem/annexes/0_overview.mdx in the wiki documents the annex API and needs a matching entry. That is a separate repository, so it is not bundled here.

Closes #483