Subaudit/update_pltbind.c contains unreachable code

Nobody has claimed this yet.

**Priority: Low**

Spindle has two interception paths: the standard `LD_AUDIT` path (used on x86_64 and aarch64) and the SubAudit path (`OPT_SUBAUDIT`) which has only remaining use case of ppc64le.

### Bug: `spindleint_none` is an unreachable enum state.

https://github.com/llnl/Spindle/blob/0380a7cce78a4d654b839af9c191e671dab7ef14/src/client/subaudit/update_pltbind.c#L213
Sets `has_spindleint` to either `spindleint_present` or `spindleint_unset`, but
https://github.com/llnl/Spindle/blob/0380a7cce78a4d654b839af9c191e671dab7ef14/src/client/subaudit/update_pltbind.c#L231-L232
checks for `spindleint_none`.

Because has_spindleint is never set to spindleint_none, two things follow:

1. getenv("LD_PRELOAD") + strstr is called on every la_activity event for the lifetime of the process, rather than being cached after the first check.
2. The redirect_interceptions() call at line 231 is unreachable dead code — spindleint_none is defined in the enum and has a consumer, but is never assigned.

Fix:
```
has_spindleint = ld_preload && strstr(ld_preload, "libspindleint.so") ? spindleint_present : spindleint_none;
```

Potential follow-up:

- Confirm whether redirect_interceptions() / spindleint_none represents intentional design or can be removed.

Start in src/client/subaudit/update_pltbind.c at the has_spindleint assignment around line 213 and the spindleint_none check around lines 231-232. Trace the la_activity path and confirm that the enum assignment makes the cache and redirect_interceptions() branch reachable. Done means the state handling matches the intended behavior and the unreachable-code and repeated LD_PRELOAD lookup issues are resolved.

Written by the indexing model from the issue text.