Real-WordPress PHP smoke harness needs a deterministic bootstrap contract

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## Problem
The real-WordPress smoke execution environment can have WordPress functions loaded without the smoke being able to rely on the usual bootstrap markers. In Data Machine PR CI, a pure-PHP smoke guarded on `defined( 'WPINC' )`, but WP functions such as `do_action()` were already loaded, causing:

`Cannot redeclare do_action() (previously declared in /wordpress/wp-includes/plugin.php)`

This required changing the smoke to guard every stub with `function_exists()`. That is reasonable defensively, but the harness contract is ambiguous.

## Expected
WP Codebox should document and/or enforce a deterministic contract for real-WordPress PHP smoke execution:

- whether WordPress is fully bootstrapped
- whether `WPINC`/`ABSPATH` are defined
- whether core functions are loaded
- whether pure-PHP smoke files should be skipped, isolated, or run in a non-WP process

## Why
Ambiguous partial bootstrap makes standalone smoke tests brittle and creates CI-only failures that are hard to reproduce locally.

## Evidence
Data Machine PR: https://github.com/Extra-Chill/data-machine/pull/2638
Initial failure: `tests/adjacent-handler-tool-policy-smoke.php` redeclared `do_action()` under WP Codebox real-WP host smoke execution.

Inizia tracciando il percorso di esecuzione dello smoke test dell’host WordPress reale e leggi tests/adjacent-handler-tool-policy-smoke.php insieme alle evidenze del PR di Data Machine. Determina se l’harness viene avviato completamente, quali marker e funzioni del core sono disponibili e se gli smoke test in puro PHP vengono saltati, isolati o eseguiti separatamente. Il lavoro è completato quando il contratto è documentato o applicato e il fallimento dovuto alla redeclaration durante un bootstrap parziale è coperto.

Scritto dal modello di indicizzazione a partire dal testo della issue.