fix: catch duplicate declarations within a batch of snippets

[TallblokeUK]

Stacked on #491. That branch is the base, so review it first — this diff shows only the batch change once #491 lands.

Addresses part of #230, and it is worse than that issue describes: this does not merely allow duplicate names, it takes the site down and locks you out of the admin.

The bug

Select two snippets that declare the same function, choose Activate, press Apply. Both activate. The next request fatals.

Each snippet is validated on its own, against whatever PHP has declared at that moment. That set does not include the other snippets in the same batch, so neither sees the other and both pass.

Fatal error: Cannot redeclare function cs_duplicate_helper()
… in snippet-ops.php(778) : eval()'d code on line 1

Why this is not a warning-level problem

The redeclaration happens at runtime inside eval(), where it is a fatal error, not a catchable one. execute_snippet() wraps execution in try/catch ( Throwable ) and the catch never fires — I checked.

Measured on WP 7.1:

	before	after
Front page	500	200
wp-admin	500	302
Snippets screen	500	reachable

So you cannot get to the snippets screen to switch one off. And ?snippets-safe-mode=1, the documented escape hatch, is itself broken until #484 lands — the two compound.

The fix

Carry the identifiers claimed by accepted snippets through the batch. The second snippet to claim a name is held back rather than activated alongside the first.

Deliberately conservative:

• Guarded declarations still activate. A function_exists() check is already recorded as an exception and cannot fatal.
• Anonymous functions claim nothing, so add_filter( …, function () {} ) is unaffected.
• Scripts and stylesheets are not considered, sharing no namespace with PHP.
• No code is rewritten. Nothing is renamed or namespaced; a snippet is simply not activated.

Testing

6 unit tests. Two fail without the fix, four are guards:

test	without fix
Two snippets declaring the same function	fails
Two snippets declaring the same class	fails
Different names both activate	passes (guard)
Guarded declarations both activate	passes (guard)
A script is not blocked by a PHP name	passes (guard)
Anonymous functions do not collide	passes (guard)

Verified end to end as above. Full suite 170 tests, phpcs clean.

What this does not solve

The protection remains point in time. A snippet that is safe today still fatals if a plugin installed next month declares the same name, and nothing re-checks. On a modest install there are already over 5,600 declared function names, most of them from plugins, so that is a live risk rather than a theoretical one.

Options discussed, none free:

• Re-validate on plugin activation and hold back snippets that would now fatal. Closes the gap, but silently disabling someone's code needs care.
• Encourage function_exists() guards, which the validator already honours. Converts a fatal into a snippet that quietly does not redeclare.
• Prefix or namespace declarations automatically. Attractive, but rewriting names cannot be done safely: a function name appearing in a string is indistinguishable from data, so add_action( 'init', 'my_func' ) and $label = 'my_func'; cannot be told apart. I tested this and it silently corrupts working snippets, which is worse than the loud failure it replaces.

Worth deciding on #230 rather than here.